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()`.
diff --git a/programs/ssl/ssl_fork_server.c b/programs/ssl/ssl_fork_server.c
index 80407e4..1ff5369 100644
--- a/programs/ssl/ssl_fork_server.c
+++ b/programs/ssl/ssl_fork_server.c
@@ -54,14 +54,14 @@
            "MBEDTLS_NET_C and/or MBEDTLS_RSA_C and/or "
            "MBEDTLS_CTR_DRBG_C and/or MBEDTLS_X509_CRT_PARSE_C and/or "
            "MBEDTLS_TIMING_C and/or MBEDTLS_PEM_PARSE_C not defined.\n");
-    return( 0 );
+    mbedtls_exit( 0 );
 }
 #elif defined(_WIN32)
 int main( void )
 {
     mbedtls_printf("_WIN32 defined. This application requires fork() and signals "
            "to work correctly.\n");
-    return( 0 );
+    mbedtls_exit( 0 );
 }
 #else
 
@@ -416,7 +416,7 @@
     fflush( stdout ); getchar();
 #endif
 
-    return( exit_code );
+    mbedtls_exit( exit_code );
 }
 #endif /* MBEDTLS_BIGNUM_C && MBEDTLS_CERTS_C && MBEDTLS_ENTROPY_C &&
           MBEDTLS_SSL_TLS_C && MBEDTLS_SSL_SRV_C && MBEDTLS_NET_C &&