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_mail_client.c b/programs/ssl/ssl_mail_client.c
index 3163e21..08ff025 100644
--- a/programs/ssl/ssl_mail_client.c
+++ b/programs/ssl/ssl_mail_client.c
@@ -56,7 +56,7 @@
            "MBEDTLS_NET_C and/or MBEDTLS_RSA_C and/or "
            "MBEDTLS_CTR_DRBG_C and/or MBEDTLS_X509_CRT_PARSE_C "
            "not defined.\n");
-    return( 0 );
+    mbedtls_exit( 0 );
 }
 #else
 
@@ -853,7 +853,7 @@
     fflush( stdout ); getchar();
 #endif
 
-    return( exit_code );
+    mbedtls_exit( exit_code );
 }
 #endif /* MBEDTLS_BIGNUM_C && MBEDTLS_ENTROPY_C && MBEDTLS_SSL_TLS_C &&
           MBEDTLS_SSL_CLI_C && MBEDTLS_NET_C && MBEDTLS_RSA_C **