x11-wm/dwm: New option, which adds st and dmenu to runtime dependencies

This patch adds an option to the dwm port, which makes x11/dmenu
and x11/sterm be runtime dependencies of dwm. The option is enabled
by default so that it's useful to users depending on pkg.

While here:

• Remove unused PATCH_SITES.
• Silence STRIP_CMD.
• Remove now outdated pkg-message.

PR: 227966
Reported by: wkoszek
Reviewed by: krion (mentor)
Approved by: krion (mentor)
Differential Revision: https://reviews.freebsd.org/D16358