Unify the example programs' termination

This is done to account for platforms, for which we want custom behavior
upon the program termination, hence we call `mbedtls_exit()` instead of
returning from `main()`.

For the sake of consistency, introduces the modifications have been made
to the test and utility examples as well. These, while less likely to be
used in the low level environments, won't suffer from such a change.
diff --git a/programs/x509/cert_app.c b/programs/x509/cert_app.c
index c57ecca..146759e 100644
--- a/programs/x509/cert_app.c
+++ b/programs/x509/cert_app.c
@@ -34,6 +34,7 @@
 #define mbedtls_time_t          time_t
 #define mbedtls_fprintf         fprintf
 #define mbedtls_printf          printf
+#define mbedtls_exit            exit
 #define MBEDTLS_EXIT_SUCCESS    EXIT_SUCCESS
 #define MBEDTLS_EXIT_FAILURE    EXIT_FAILURE
 #endif /* MBEDTLS_PLATFORM_C */
@@ -50,7 +51,7 @@
            "MBEDTLS_NET_C and/or MBEDTLS_RSA_C and/or "
            "MBEDTLS_X509_CRT_PARSE_C and/or MBEDTLS_FS_IO and/or "
            "MBEDTLS_CTR_DRBG_C not defined.\n");
-    return( 0 );
+    mbedtls_exit( 0 );
 }
 #else
 
@@ -491,7 +492,7 @@
     fflush( stdout ); getchar();
 #endif
 
-    return( exit_code );
+    mbedtls_exit( exit_code );
 }
 #endif /* MBEDTLS_BIGNUM_C && MBEDTLS_ENTROPY_C && MBEDTLS_SSL_TLS_C &&
           MBEDTLS_SSL_CLI_C && MBEDTLS_NET_C && MBEDTLS_RSA_C &&
diff --git a/programs/x509/cert_req.c b/programs/x509/cert_req.c
index 784f719..9eb64b5 100644
--- a/programs/x509/cert_req.c
+++ b/programs/x509/cert_req.c
@@ -31,6 +31,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #define mbedtls_printf          printf
+#define mbedtls_exit            exit
 #define MBEDTLS_EXIT_SUCCESS    EXIT_SUCCESS
 #define MBEDTLS_EXIT_FAILURE    EXIT_FAILURE
 #endif /* MBEDTLS_PLATFORM_C */
@@ -45,7 +46,7 @@
             "MBEDTLS_PK_PARSE_C and/or MBEDTLS_SHA256_C and/or "
             "MBEDTLS_ENTROPY_C and/or MBEDTLS_CTR_DRBG_C "
             "not defined.\n");
-    return( 0 );
+    mbedtls_exit( 0 );
 }
 #else
 
@@ -436,7 +437,7 @@
     fflush( stdout ); getchar();
 #endif
 
-    return( exit_code );
+    mbedtls_exit( exit_code );
 }
 #endif /* MBEDTLS_X509_CSR_WRITE_C && MBEDTLS_PK_PARSE_C && MBEDTLS_FS_IO &&
           MBEDTLS_ENTROPY_C && MBEDTLS_CTR_DRBG_C && MBEDTLS_PEM_WRITE_C */
diff --git a/programs/x509/cert_write.c b/programs/x509/cert_write.c
index adade1c..1ab16ec 100644
--- a/programs/x509/cert_write.c
+++ b/programs/x509/cert_write.c
@@ -31,6 +31,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #define mbedtls_printf          printf
+#define mbedtls_exit            exit
 #define MBEDTLS_EXIT_SUCCESS    EXIT_SUCCESS
 #define MBEDTLS_EXIT_FAILURE    EXIT_FAILURE
 #endif /* MBEDTLS_PLATFORM_C */
@@ -46,7 +47,7 @@
             "MBEDTLS_FS_IO and/or MBEDTLS_SHA256_C and/or "
             "MBEDTLS_ENTROPY_C and/or MBEDTLS_CTR_DRBG_C and/or "
             "MBEDTLS_ERROR_C not defined.\n");
-    return( 0 );
+    mbedtls_exit( 0 );
 }
 #else
 
@@ -813,7 +814,7 @@
     fflush( stdout ); getchar();
 #endif
 
-    return( exit_code );
+    mbedtls_exit( exit_code );
 }
 #endif /* MBEDTLS_X509_CRT_WRITE_C && MBEDTLS_X509_CRT_PARSE_C &&
           MBEDTLS_FS_IO && MBEDTLS_ENTROPY_C && MBEDTLS_CTR_DRBG_C &&
diff --git a/programs/x509/crl_app.c b/programs/x509/crl_app.c
index f831683..3c11e90 100644
--- a/programs/x509/crl_app.c
+++ b/programs/x509/crl_app.c
@@ -31,6 +31,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #define mbedtls_printf          printf
+#define mbedtls_exit            exit
 #define MBEDTLS_EXIT_SUCCESS    EXIT_SUCCESS
 #define MBEDTLS_EXIT_FAILURE    EXIT_FAILURE
 #endif /* MBEDTLS_PLATFORM_C */
@@ -41,7 +42,7 @@
 {
     mbedtls_printf("MBEDTLS_BIGNUM_C and/or MBEDTLS_RSA_C and/or "
            "MBEDTLS_X509_CRL_PARSE_C and/or MBEDTLS_FS_IO not defined.\n");
-    return( 0 );
+    mbedtls_exit( 0 );
 }
 #else
 
@@ -145,7 +146,7 @@
     fflush( stdout ); getchar();
 #endif
 
-    return( exit_code );
+    mbedtls_exit( exit_code );
 }
 #endif /* MBEDTLS_BIGNUM_C && MBEDTLS_RSA_C && MBEDTLS_X509_CRL_PARSE_C &&
           MBEDTLS_FS_IO */
diff --git a/programs/x509/req_app.c b/programs/x509/req_app.c
index 0f20c85..0ddece4 100644
--- a/programs/x509/req_app.c
+++ b/programs/x509/req_app.c
@@ -31,6 +31,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #define mbedtls_printf          printf
+#define mbedtls_exit            exit
 #define MBEDTLS_EXIT_SUCCESS    EXIT_SUCCESS
 #define MBEDTLS_EXIT_FAILURE    EXIT_FAILURE
 #endif /* MBEDTLS_PLATFORM_C */
@@ -41,7 +42,7 @@
 {
     mbedtls_printf("MBEDTLS_BIGNUM_C and/or MBEDTLS_RSA_C and/or "
            "MBEDTLS_X509_CSR_PARSE_C and/or MBEDTLS_FS_IO not defined.\n");
-    return( 0 );
+    mbedtls_exit( 0 );
 }
 #else
 
@@ -145,7 +146,7 @@
     fflush( stdout ); getchar();
 #endif
 
-    return( exit_code );
+    mbedtls_exit( exit_code );
 }
 #endif /* MBEDTLS_BIGNUM_C && MBEDTLS_RSA_C && MBEDTLS_X509_CSR_PARSE_C &&
           MBEDTLS_FS_IO */