stty: diagnose an invalid hex value in 35-colon commmand-line argument

stty: diagnose an invalid hex value in 35-colon commmand-line argument
* NEWS: Mention this.
* src/stty.c (strtoul_tcflag_t, strtoul_cc_t): New functions.
(recover_mode): Use those functions (not sscanf), to parse the
string robustly.
* tests/stty/invalid: New file.  Test for the above.
* tests/stty/Makefile.am (TESTS): Add invalid.
* .x-sc_prohibit_atoi_atof: Don't exempt stty.c from this check.
Add tests/stty/invalid so we don't have to obfuscate the comment
about sscanf therein.
* Makefile.maint (sc_prohibit_atoi_atof): Mention sscanf in the
diagnostic, too.