Make procstat(1) recognize process descriptors, so that it shows
"P" instead of "?" in "procstat -af" output. Note that there are
still a few more DTYPE_* kinds we don't decode yet.

Reported by: rwatson
MFC after: 2 weeks
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D12426