bmake: Add a package for bmake and share/mk

bmake along with /usr/share/mk seems to meet the criteria for being in
its own package: it's self-contained and the majority of users (who are
using binary packages) don't need to install it, so it doesn't belong in
FreeBSD-utilities. Instead, put it in its own package.

Add a dependency on the new package from:

• sendmail (uses make to build its configuration)
• NIS (uses make to build the database).

But not from examples, because while some of the examples do require
make, they are intended as examples rather than something to be used
directly.

Reviewed by: manu, sjg
Differential Revision: https://reviews.freebsd.org/D50149