Add a tunable to set the pgcache zone's maxcache

When it is set to 0 (the default), a heavy Netflix-style web workload
suffers from heavy lock contention on the vm page free queue called from
vm_page_zone_{import,release}() as the buckets are frequently drained.
When setting the maxcache, this contention goes away.

We should eventually try to autotune this, as well as make this
zone eligable for uma_reclaim().

Reviewed by: alc, markj
Not Objected to by: jeff
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D22112