logger: use xgetlogin from pwdutils.

logger: use xgetlogin from pwdutils.

It defined its own xgetlogin, which queried geteuid() instead of getuid(),
with a fallback to "<someone>" when lookup fails. This has been the case
since the function was introduced in
019b97024fde3f07eaf541eef990762483369a11, so geteuid() has always been
used. Since using geteuid for identification isn't consistent with the
rest of util-linux, switching to xgetlogin(), which uses getuid(),
should be correct.

Signed-off-by: Érico Nogueira <erico.erc@gmail.com>