[SV 57930] Cast char to unsigned char to call ctype functions

[SV 57930] Cast char to unsigned char to call ctype functions

This cast was already done almost everywhere: fix some stragglers.

* src/load.c (load_file): Cast char to unsigned char.
* src/misc.c (strcasecmp, strncasecmp): [!POSIX] Ditto.
* src/dir.c (vms_hash): [VMS] Ditto.
* src/vms_progname.c (set_program_name): [VMS] Ditto.
* src/vms_jobs.c (posix_parse_dq): [VMS] Ditto.
(posix_parse_dollar): [VMS] Ditto.
(build_vms_cmd): [VMS] Ditto.
(child_execute_job): [VMS] Ditto.