The very common case for this function is that during AUTH EXTERNAL,
it receives a Unix uid encoded as an ASCII decimal integer. There is
no need to look up such uids in the system's user database
(/etc/password or NSS) when the only information we are going to use
from the DBusUserInfo struct is the uid anyway. This avoids taking
the lock and performing a potentially time-consuming NSS lookup.

This changes behaviour in one corner case: if a privileged process has
used one of the set*uid family of functions to set its effective uid
to a numeric uid that does not exist in the system's user database,
we would previously fail. Now, we succeed anyway: it is true to say
in the DBusCredentials that the process has uid 12345, even if uid
12345 does not correspond to any named user.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Philip Withnall <withnall@endlessm.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=104588