Show wiring state of map entries in procstat -v.

Note that only entries wired by userspace are shown as such. In
particular, entries transiently wired by sysctl_wire_old_buffer() are
not flagged as wired in procstat -v output.

Reviewed by: kib (previous version)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D19461