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.

scripts/hash_burn.c

@@ -43,8 +43,7 @@ int main(int argc, char *argv[])
 		limit = atoi(argv[2]);
 	}
 
-	alg = enum_from_name(hash_algorithm_short_names, argv[1]);
-	if (alg == -1)
+	if (!enum_from_name(hash_algorithm_short_names, argv[1], &alg))
 	{
 		fprintf(stderr, "unknown hash algorthm: %s\n", argv[1]);
 		return 1;