Make unit_name_to_instance() return UnitNameFlags

The function returns non-negative UnitNameFlags on success, and negative
errno on error. In the past we kept the return type as int because of those
negative return values. But nowadays _UNIT_NAME_INVALID == -EINVAL. And if
we tried to actually return something that doesn't fit in the return type,
the compiler would throw an error. By changing to the "real" return type,
we allow the debugger to use symbolic representation for the variables.

src/basic/unit-file.c

@@ -72,7 +72,7 @@ int unit_validate_alias_symlink_and_warn(const char *filename, const char *targe
         const char *src, *dst;
         _cleanup_free_ char *src_instance = NULL, *dst_instance = NULL;
         UnitType src_unit_type, dst_unit_type;
-        int src_name_type, dst_name_type;
+        UnitNameFlags src_name_type, dst_name_type;
 
         /* Check if the *alias* symlink is valid. This applies to symlinks like
          * /etc/systemd/system/dbus.service → dbus-broker.service, but not to .wants or .requires symlinks