Uses/display.mk: Overhaul the module

[arrowd]

Summary

• 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.

Test Plan

This fixes lang/seed7 breakage introduced with my previous commit
https://lists.freebsd.org/archives/dev-commits-ports-all/2025-October/177617.html

I also tested it against x11-toolkits/gtk30.

[bapt]

Mk/Uses/display.mk

33

in my opinion we shoud reject all arguments but build and test until someone comes with a use case for other situations
this should become something like

.if ${display_args:Mbuild} || empty(display_args)
MAKE_ENV+= DISPLAY...
.elif ${display_args:Mtest}
TEST_ENV:+ DISPLAY...
.else
.error invalid argument
.endif

DISPLAY_DEPENDS= ${dipslay_ARGS:U}_DEPENDS

[arrowd]

• Remove some targets

[arrowd]

Mk/Uses/display.mk

33

It is not possible to do DISPLAY_DEPENDS= ${dipslay_ARGS:U}_DEPENDS, because dipslay_ARGS may have multiple words.