genfstab: make -p default behavior, add -P as inverse

This should be the default, as you generally don't want to include
pseudofs mounts in your /etc/fstab.
genfstab: strip seclabel from optstring

People seem to suffer from this, so let's just fix it.

https://www.marshut.net/krqsrn/genfstab-fstab-seclabel-uuid-vs-partuuid-and-arch-wiki.html
https://bbs.archlinux.org/viewtopic.php?pid=1360580