namespace-util: Merge namespace_enter_delegated() into namespace_enter()

There's no need to pass in a boolean to decide whether we use
namespace_enter_delegated() or not. Instead, we can just check if we
have CAP_SYS_ADMIN in our own user namespace. If we don't, then we have
to insist on a child user namespace being passed in and we have to enter
it first to get CAP_SYS_ADMIN as without CAP_SYS_ADMIN we wouldn't be able
to call setns() in the first place. If we do have CAP_SYS_ADMIN, we can
always enter the other namespaces first before entering the user namespace.

Additionally, we don't fail anymore if we can't reset the UID/GID since a
root user might not always be available in every user namespace we might
enter.

src/basic/process-util.c

@@ -1778,7 +1778,6 @@ int namespace_fork_full(
                 int netns_fd,
                 int userns_fd,
                 int root_fd,
-                bool delegated,
                 PidRef *ret) {
 
         _cleanup_(pidref_done_sigkill_wait) PidRef pidref_outer = PIDREF_NULL;
@@ -1824,10 +1823,7 @@ int namespace_fork_full(
 
                 errno_pipe_fd[0] = safe_close(errno_pipe_fd[0]);
 
-                if (delegated)
-                        r = namespace_enter_delegated(userns_fd, pidns_fd, mntns_fd, netns_fd, root_fd);
-                else
-                        r = namespace_enter(pidns_fd, mntns_fd, netns_fd, userns_fd, root_fd);
+                r = namespace_enter(pidns_fd, mntns_fd, netns_fd, userns_fd, root_fd);
                 if (r < 0) {
                         log_full_errno(prio, r, "Failed to join namespace: %m");
                         report_errno_and_exit(errno_pipe_fd[1], r);