add a shm example

Merge fstab.tmpfs into fstab.ramdisk so that the file names represent
concepts rather than specific file systems. Suggested by hubertf@.

Fix typo: ammount -> amount.

No devices on /usr -> mount -o nodev
No setuid programs in /var -> mount -o nodev,nosuid

Adding "noexec" in various places may cause too much damage
(e.g. for running DEINSTALL scripts from /var/db

No devices on /usr -> mount -o nodev
No setuid programs in /var -> mount -o nodev,nosuid

Adding "noexec" in various places may cause too much damage
(e.g. for running DEINSTALL scripts from /var/db/pkg, configure
scripts, etc).

Inspired by OpenBSD's afterboot(8) manpage.

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Extract commonly used things like CDROMs, pseudo-filesystems (/proc, ...)
and mfs-based /tmp into their own files. Hint at them in the existing
files. (fstab needs an #include statement :)