var_run: Clean up style

Clean up style and make more consistent.

Replace test with if-then-else to make the script more legible.

Replace the call to dirname with the shell %/* operator avoiding a
fork & exec.

Reorder the test for $var_run_autosave before the test for /var/run on
tmpfs. This avoids gratuitously scanning the mount table for a tmpfs
/var/run.

Initial concept by and in discussion with:
Harry Schmalzbauer <freebsd@omnilan.de>

No functional change intended.

MFC after: 2 weeks
Differnential revision: https://reviews.freebsd.org/D47773