enum: Return boolean result for enum_from_name() lookup

Handling the result for enum_from_name() is difficult, as checking for
negative return values requires a cast if the enum type is unsigned. The new
signature clearly differentiates lookup result from lookup value.

Further, this actually allows to convert real -1 enum values, which could not
be distinguished from "not-found" and the -1 return value.

This also fixes several clang warnings where enums are unsigned.

src/libtls/tls_crypto.c

@@ -959,8 +959,8 @@ static void filter_specific_config_suites(private_tls_crypto_t *this,
 			enumerator = enumerator_create_token(config, ",", " ");
 			while (enumerator->enumerate(enumerator, &token))
 			{
-				suite = enum_from_name(tls_cipher_suite_names, token);
-				if (suite == suites[i].suite)
+				if (enum_from_name(tls_cipher_suite_names, token, &suite) &&
+					suite == suites[i].suite)
 				{
 					suites[remaining++] = suites[i];
 					break;