Make vmdaemon timeout configurable

Make vmdaemon timeout configurable, so that one can adjust
how often it runs.

Here's a trick: set this to 1, then run 'limits -m 0 sh',
then run whatever you want with 'ktrace -it XXX', and observe
how the working set changes over time.

Reviewed By: kib
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D22038