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.

programs/pkey/rsa_encrypt.c

@@ -55,7 +55,7 @@ int main( void )
     mbedtls_printf("MBEDTLS_BIGNUM_C and/or MBEDTLS_RSA_C and/or "
            "MBEDTLS_ENTROPY_C and/or MBEDTLS_FS_IO and/or "
            "MBEDTLS_CTR_DRBG_C not defined.\n");
-    return( 0 );
+    mbedtls_exit( 0 );
 }
 #else
 int main( int argc, char *argv[] )
@@ -182,7 +182,7 @@ exit:
     fflush( stdout ); getchar();
 #endif
 
-    return( exit_code );
+    mbedtls_exit( exit_code );
 }
 #endif /* MBEDTLS_BIGNUM_C && MBEDTLS_RSA_C && MBEDTLS_ENTROPY_C &&
           MBEDTLS_FS_IO && MBEDTLS_CTR_DRBG_C */