Don't abort startup when setgid/setuid() fails with EINVAL

Don't abort startup when setgid/setuid() fails with EINVAL

Both setgid(2) as well as setuid(2) can fail with EINVAL in addition to
EPERM, their manual pages state "EINVAL: The user/group ID specified in
uid/gid is not valid in this user namespace ".

So not only treat EPERM as an "acceptable error" and continue with
logging the error, but do the same for EINVAL.

This was triggered by the Void Linux xbps-uunshare(1) tool used for
building "XBPS source packages" and reported by luca in #ngircd. Thanks!