Accept NSS aliases for canonicalized user records (#42452)

This PR fixes userdb lookups for NSS users that are resolved through an
alias but returned with a canonical user name.

Some NSS providers, such as SSSD, can successfully resolve a user by an
alias-like name, for example a Kerberos/AD UPN (for example
testuser@example.test) while returning a passwd record with the
canonical login name.

The original lookup name was not preserved. Later, the userdb worker
checked whether the returned record matched the requested name with
user_record_matches_user_name(). Since the requested name was
testuser@example.test, but the record only contained testuser, the
lookup was rejected as:
`io.systemd.UserDatabase.ConflictingRecordFound`

This also caused pam_systemd to fail opening sessions for such users
with:
`pam_systemd(...:session): Failed to get user record`

In my case, this broke graphical logins for Samba users logging in with
a UPN on systems where SSSD canonicalizes the NSS result.

My solution preserves the requested name as a UserRecord alias when an
NSS lookup by name succeeds but the returned pw_name differs from the
requested name.
This allows user_record_matches_user_name() to accept the canonicalized
NSS result instead of treating it as a conflicting record.
The patch does not invent new aliases. It only records the name that NSS
itself already accepted and resolved successfully.

I'm not an expert on working with systemd, so I'm asking for a review of
my PR.

diff --cc src/test/meson.build
Simple merge