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/ssl/ssl_pthread_server.c b/programs/ssl/ssl_pthread_server.c
index 9a05ad8..b51bea6 100644
--- a/programs/ssl/ssl_pthread_server.c
+++ b/programs/ssl/ssl_pthread_server.c
@@ -30,9 +30,11 @@
 #include "mbedtls/platform.h"
 #else
 #include <stdio.h>
+#include <stdlib.h>
 #define mbedtls_fprintf    fprintf
 #define mbedtls_printf     printf
 #define mbedtls_snprintf   snprintf
+#define mbedtls_exit       exit
 #endif
 
 #if !defined(MBEDTLS_BIGNUM_C) || !defined(MBEDTLS_CERTS_C) ||            \
@@ -50,7 +52,7 @@
            "MBEDTLS_CTR_DRBG_C and/or MBEDTLS_X509_CRT_PARSE_C and/or "
            "MBEDTLS_THREADING_C and/or MBEDTLS_THREADING_PTHREAD "
            "and/or MBEDTLS_PEM_PARSE_C not defined.\n");
-    return( 0 );
+    mbedtls_exit( 0 );
 }
 #else
 
@@ -520,7 +522,7 @@
     fflush( stdout ); getchar();
 #endif
 
-    return( ret );
+    mbedtls_exit( ret );
 }
 
 #endif /* MBEDTLS_BIGNUM_C && MBEDTLS_CERTS_C && MBEDTLS_ENTROPY_C &&