Change NUL char handling of isspecial()

Change NUL char handling of isspecial()

Replace isspecial() by the new macro is_glob_special(), which is more,
well, specialized.  The former included the NUL char in its character
class, while the letter only included characters that are special to
file name globbing.

The new name contains underscores because they enhance readability
considerably now that it's made up of three words.  Renaming the
function is necessary to document its changed scope.

The call sites of isspecial() are updated to check explicitly for NUL.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>