zshenv: kill off legacy X11 paths.

Debian moved to X11R7 back in 2006, and presumably other distributions
also did around the same time.  There's no need to support the legacy
/usr/X11R6 and /usr/bin/X11 paths, as no modern OS uses them.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
zshenv: set LC_MEASUREMENT to POSIX.

The en_US.UTF-8 setting is 2 (US customary) and POSIX is 1 (metric).  We
prefer the latter, so use POSIX.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
zshenv: update GROFF_TMAC_PATH.

The TenorSAX resources have been split out into a separate repository,
so refer to the new path.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
zshenv: honor external locale settings.

If the value of LANG we get from the process that spawned the shell is
English, use American English for certain locale settings; otherwise,
use Spanish or French if that's set, and fall back to American English.
This only applies for text; for non-text, always use the default
settings.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>