Fix log uid for logrotate-3.8 compat (bug 378451)

Fix log uid for logrotate-3.8 compat (bug 378451)

If PORT_LOGDIR is writable by the portage group but its uid is not
portage_uid, then set the uid to portage_uid if we have privileges to
do so, and also copy the uid to the logfile. This fixes logrotate
chown failures during the compression phase, when it attempts to copy
the uid from the logfile to a temp file. With the "su portage portage"
directive and logrotate-3.8.0, logrotate's chown call during the
compression phase will only succeed if the log file's uid is
portage_uid.