lib: s/igt_env_set/igt_check_boolean_env_var

lib: s/igt_env_set/igt_check_boolean_env_var

So I wasn't really happy with env_set since it's way too close to
setenv(), whic actually _sets_ and environment variable. So use check
instead of set as the verb (well adjective for env_set).

Also sprinkle in some hints that we talk about a boolean setting here.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>