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_pthread_server.c b/programs/ssl/ssl_pthread_server.c
index 0624d33..3496ef6 100644
--- a/programs/ssl/ssl_pthread_server.c
+++ b/programs/ssl/ssl_pthread_server.c
@@ -54,7 +54,7 @@
            "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
 
@@ -524,7 +524,7 @@
     fflush( stdout ); getchar();
 #endif
 
-    return( ret );
+    mbedtls_exit( ret );
 }
 
 #endif /* MBEDTLS_BIGNUM_C && MBEDTLS_CERTS_C && MBEDTLS_ENTROPY_C &&