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/charon-cmd/cmd/cmd_connection.c

@@ -460,10 +460,9 @@ static void add_ts(private_cmd_connection_t *this,
  */
 static void set_profile(private_cmd_connection_t *this, char *name)
 {
-	int profile;
+	profile_t profile;
 
-	profile = enum_from_name(profile_names, name);
-	if (profile == -1)
+	if (!enum_from_name(profile_names, name, &profile))
 	{
 		DBG1(DBG_CFG, "unknown connection profile: %s", name);
 		exit(1);