Fix arc after r326347 broke various memory limit queries. Use UMA features
rather than kmem arena size to determine available memory.

Initialize the UMA limit to LONG_MAX to avoid spurious wakeups on boot before
the real limit is set.

PR: 224330 (partial), 224080
Reviewed by: markj, avg
Sponsored by: Netflix / Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D13494