namespace-util: introduce userns_acquire_self_root()

This is a simple helper for creating a userns that just maps the
callers user to UID 0 in the namespace. This can be acquired unpriv,
which makes it useful for various purposes, for example for the logic in
is_idmapping_supported(), hence port it over.

(is_idmapping_supported() used a different mapping before, with the
nobody users, but there's no real reason for that, and we'll use
userns_acquire_self_root() elsewhere soon, where the root mapping is
important).

src/basic/namespace-util.h

@@ -87,6 +87,7 @@ int parse_userns_uid_range(const char *s, uid_t *ret_uid_shift, uid_t *ret_uid_r
 
 int userns_acquire_empty(void);
 int userns_acquire(const char *uid_map, const char *gid_map, bool setgroups_deny);
+int userns_acquire_self_root(void);
 int userns_enter_and_pin(int userns_fd, pid_t *ret_pid);
 
 int userns_get_base_uid(int userns_fd, uid_t *ret_uid, gid_t *ret_gid);