Fix signedness of char values passed to isspace, iscntrl, isxdigit

Fix signedness of char values passed to isspace, iscntrl, isxdigit
(eliminate 'array subscript has type char' GCC warning)

* cord/tests/de_win.c (WinMain, plain_chars, control_chars): Cast char
value to unsigned char when passed to isspace(), iscntrl().
* os_dep.c (GC_parse_map_entry): Change type of "start_start",
"end_start", "maj_dev_start", "p" local variables from char* to
unsgined char* (as isspace(), isxdigit() expect non-negative values);
insert casts between char* and unsigned char* where needed.
* os_dep.c (GC_parse_map_entry): Remove "endp" local variable (use "p"
variable directly instead).