Add new stats(7) man page and hook it up to the build.

This man page contains stat utilities that are available in
the base system. This is a better approach than looking them
up via "apropos stat" or similar commands.

Thanks to Daniel Ebdrup Jensen for writing the original page
and incorporating the feedback given.

Submitted by: Daniel Ebdrup Jensen
Reviewed by: 0mp, allanjude, brueffer, bcr
Approved by: bcr
Relnotes: yes (new stats(7) man page)
Differential Revision: https://reviews.freebsd.org/D24417