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/pki/commands/self.c

@@ -95,8 +95,7 @@ static int self()
 				}
 				continue;
 			case 'g':
-				digest = enum_from_name(hash_algorithm_short_names, arg);
-				if (digest == -1)
+				if (!enum_from_name(hash_algorithm_short_names, arg, &digest))
 				{
 					error = "invalid --digest type";
 					goto usage;