fstat: identify kqueue, mqueue, and procdesc file descriptors

When displaying kqueue, mqueue, and procdesc file descriptors, identify
them as such. There aren't any details, but it's better than a scary
warning like "unknown file type 15 for file 6 of pid 26393".

Sponsored by: ConnectWise
Differential Revision: https://reviews.freebsd.org/D48706

(cherry picked from commit b3168306a13b9d95c2b01bf7a483c1af9ec0d797)