main/openrc: default mount /dev noexec

Users who still need to be able to exec from /dev can use the following fstab entry to override the default

devtmpfs /dev devtmpfs exec,nosuid,mode=0755 0 0

Since I read this https://www.phoronix.com/scan.php?page=news_item&px=Linux-5.17-Devtmpfs-Change I've been running with /dev mounted noexec without any issues.