Simplify unexec file mode setting

* unexaix.c, unexcoff.c, unexelf.c, unexmacosx.c:
Don't include <sys/stat.h> when no longer needed.
(unexec): Create file with correct mode in the first place,
rather than overwriting the mode later and fiddling with the
global umask in the mean time.  Avoid bogus usage like
'umask (777)', which should have been 'umask (0777)'.
(mark_x): Remove. All callers removed.