Rename check-man to check-man-warnings

Currently the check-man test is dependent on the MAN_HAS_WARNINGS
test, which made sense as long as the script only did the warnings
check.

However, we should run the [em] tests always, and in general we could
run other tests too, even if man doesn't support --warnings; so let's
split the check man. As a first prerequisite, we rename check-man to
check-man-warnings (and the respective Makefile variables).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>