diskless: Miscellaneous fixes.

diskless: Miscellaneous fixes.

* Increase the space reserved for /etc whose size has grown considerably
  due to the blacklists in /etc/ssh.

* Remove the check for devfs which has been around long enough.

* Use tmpfs instead of mfs.

* Pass whatever is in md_size (e.g. 10M) directly to mount_tmpfs. For
  backwards compatibility, treat number-only values as sectors (i.e.,
  multiply by 512 and pass that to mount_tmpfs).

Submitted-by:  Rumko <rumcic@gmail.com>
Dragonfly-bug: <http://bugs.dragonflybsd.org/issue1732>