Uses/display.mk: Overhaul the module
- Stop determining the free display port number. The whole block is under the !defined(DISPLAY) condition and at the time Xvfb starts the :0 port is guaranteed to be available.
- Allow multiple values in the USES args.
- Set *_ENV and *_DEPENDS more granularly.
Approved by: bapt
Differential Revision: https://reviews.freebsd.org/D53122
Fixes: 453ec167a9f355e674cd35406725e0437cef722b