uid-range: error code tweak for uid_range_load_userns()

Let's return ENOSYS if /proc/ is not mounted (as that's what we usually
return in that case in various helpers that operate on /proc/). Return
EOPNOTSUPP if the kernel simply doesn't support userns.

src/shared/uid-range.c

@@ -212,8 +212,8 @@ int uid_range_load_userns(UidRange **p, size_t *n, const char *path) {
         if (!f) {
                 r = -errno;
 
-                if (r == -ENOENT && path_startswith(path, "/proc/") && proc_mounted() > 0)
-                        return -EOPNOTSUPP;
+                if (r == -ENOENT && path_startswith(path, "/proc/"))
+                        return proc_mounted() > 0 ? -EOPNOTSUPP : -ENOSYS;
 
                 return r;
         }