Un-break peer authentication.

Commit 613c6d26 sloppily replaced a
lookup of the UID obtained from getpeereid() with a lookup of the
server's own user name, thus totally destroying peer authentication.
Revert.  Per report from Christoph Berg.

In passing, make sure get_user_name() zeroes *errstr on success on
Windows as well as non-Windows.  I don't think any callers actually
depend on this ATM, but we should be consistent across platforms.