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

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
MFC after:	3 days
Relnotes:	yes (new stats(7) man page)
Differential Revision:	https://reviews.freebsd.org/D24417