New port: x11-wm/fvwm3: New FVWM window manager

FVWM is an old window manager based on twm. After a long pause FVWM
development appears to have been restarted. This new FVWM no longer
supports M4 however it does support GO. Unfortunately fetching
go:modules doesn't work and is disabled at the moment. The maintainer
intends to work on this and upstream it when it's finally working.

Submitted by: Felix Palmen <felix@palmen-it.de>
Reviewed by: cy
Differential Revision: https://reviews.freebsd.org/D30159