Create links to the libprocstat man pages for all of the libprocstat functions so they can be looked up by name

Add a cross refrence to libprocstat in the procstat.1 man page

Differential Revision: https://reviews.freebsd.org/D2578
Suggested by: jmg
Reviewed by: brueffer
Approved by: wblock (mentor)
MFC after: 1 week
Sponsored by: ScaleEngine Inc.