Provide GNU Watch on FreeBSD

Summary:
FreeBSD doesn't offer a watch command.

Furthermore, another command to snoop on another tty
has the same name. Like coreutils, it makes sense to
be coherent to install this command as gwatch.

Test Plan: gwatch on Ysul

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2222