mail/getmail6: emergency hotfix for 6.18

A feature added in 6.18 causes problems. When trying to use all files in
--getmaildir, getmail will choke on oldmail-* files, which are likely to be
present. Since this is the new default behaviour for running getmail without
any arguments, this change is very likely to break many installations.

PR: 257024
Reported by: Michael Büker <freebsd@michael-bueker.de> (maintainer)