maint: prefer c_isxdigit when that is the intent

maint: prefer c_isxdigit when that is the intent

* src/digest.c (valid_digits, split_3):
* src/echo.c (main):
* src/printf.c (print_esc):
* src/ptx.c (unescape_string):
* src/stat.c (print_it):
When the code is supposed to support only POSIX-locale hex digits,
use c_isxdigit rather than isxdigit.  Include c-ctype.h as needed.
This defends against oddball locales where isxdigit != c_isxdigit.