- Drop one leading '.' from the filename (before adding our own).
 - Drop one trailing '.' from a (possibly truncated) name prior to
   the .XXXXXX suffix being added.
 - Allow the temp-name to collapse to just the .XXXXXX suffix
   if the path is long enough to require that.

Note that we don't try to remove multiple dots from a filename
that actually has multiple consecutive dots, since we might as
well learn early if the final name is going to fail or not.

Improved the xattrs testing to include hard-linking.

Fixes bug 8020.

This allows the same rsyncd.d directory to be used for a set
of merge files (*.inc) and a set of include files (*.conf).

The code now avoids any special internal meaning for uid/gid -1, which
allows it to be mapped to a better value (use 4294967295 instead of -1
as the ID to map).  Replaced atol() with something than can return a
value > 0x7FFFFFFF and that will error-out if the value overflows.  If
chown() is called with a uid or gid of -1, complain that the ID is not
settable and signal a transfer error.  Fixes bug 6936.

(If our POSIX types aren't valid, we can't handle the ACLs.)