uid<->username mapping cache with an entry when another entry
for that uid is already loaded. This fixes a case where the
mapping of "toor" would replace "root" when the daemon was started,
resulting in no mapping for "root" until the cache entry for "toor"
timed out.
The algorithm is inefficient, but since it is only done once when
the daemon is started up, I don't think that's an issue.
MFC after: 2 weeks
eb75badee3