Add ISDIRSEP() helper macro and use it

Add ISDIRSEP() helper macro and use it

Create a ISDIRSEP() macro to check for directory separator chars
using the stopchar_map, and replace inline checks and explicit
STOP_SET calls with this macro.

* src/makeint.h (ISDIRSEP): Create the macro using MAP_DIRSEP.
* src/dir.c (find_directory): Replace inline checks with ISDIRSEP.
(file_exists_p): Ditto.
(file_impossible): Ditto.
(file_impossible_p): Ditto.
(local_stat): Ditto.
* src/file.c (lookup_file): Ditto.
* src/function.c (abspath): Ditto.
* src/job.c (_is_unixy_shell): Ditto.
(is_bourne_compatible_shell): Ditto.
(construct_command_argv): Ditto.
* src/main.c (find_and_set_default_shell): Ditto.
(main): Ditto.
* src/read.c (eval): Ditto.
(parse_file_seq): Ditto.
* src/remake.c (name_mtime): Ditto.
* src/vpath.c (construct_vpath_list): Ditto.