posix: simplify checkexec check
Use a slightly simpler logic that in some cases can avoid an unnecessary chmod
and stat.
Instead of flipping the X bits, make it more clear that we rely on no X bits
being set on initial file creation, and that at least some of them stick after
they all have been set.
#require docutils gettext
$ $TESTDIR/check-gendoc pt_BR
checking for parse errors