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/swanctl/commands/list_certs.c

@@ -556,10 +556,10 @@ CALLBACK(list_cb, void,
 		bool has_privkey;
 
 		buf = vici_find(res, &len, "data");
-		type = enum_from_name(certificate_type_names,
-							  vici_find_str(res, "ANY", "type"));
 		has_privkey = streq(vici_find_str(res, "no", "has_privkey"), "yes");
-		if (type != -1 && type != CERT_ANY && buf)
+		if (enum_from_name(certificate_type_names,
+						   vici_find_str(res, "ANY", "type"), &type) &&
+			type != CERT_ANY && buf)
 		{
 			cert = lib->creds->create(lib->creds, CRED_CERTIFICATE, type,
 									BUILD_BLOB_ASN1_DER, chunk_create(buf, len),