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/pkey/pk_decrypt.c b/programs/pkey/pk_decrypt.c
index 1d8c959..64d9561 100644
--- a/programs/pkey/pk_decrypt.c
+++ b/programs/pkey/pk_decrypt.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 */
@@ -56,7 +57,7 @@
     mbedtls_printf("MBEDTLS_BIGNUM_C and/or MBEDTLS_PK_PARSE_C and/or "
            "MBEDTLS_FS_IO and/or MBEDTLS_ENTROPY_C and/or "
            "MBEDTLS_CTR_DRBG_C not defined.\n");
-    return( 0 );
+    mbedtls_exit( 0 );
 }
 #else
 int main( int argc, char *argv[] )
@@ -169,7 +170,7 @@
     fflush( stdout ); getchar();
 #endif
 
-    return( exit_code );
+    mbedtls_exit( exit_code );
 }
 #endif /* MBEDTLS_BIGNUM_C && MBEDTLS_PK_PARSE_C && MBEDTLS_FS_IO &&
           MBEDTLS_ENTROPY_C && MBEDTLS_CTR_DRBG_C */