var_run: Clean up style

[cy]

Summary

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 <[e-mail]>

No functional change intended.

MFC after: 2 weeks

Test Plan

Tested locally