* libc/strptime.cc: Implement support for era, alt_digits and POSIX

	* libc/strptime.cc: Implement support for era, alt_digits and POSIX
	padding and width modifiers.
	(era_info_t): New type.
	(free_era_info): New static function to free era_info_t storage.
	(get_era_info): New static function to create era_info_t storage
	from LC_TIME era information.
	(alt_digits_t): New type.
	(get_alt_digits): New static function to create alt_digits_t storage
	from LC_TIME alt_digits information.
	(free_alt_digits): New static function to free alt_digits_t storage.
	(find_alt_digits): New static function to scan input for alternative
	digits and return them, if any.  Return NULL otherwise.
	(__strptime): New static function taking all code from strptime.
	Implement handling for E, O, +, 0, and width modifiers per POSIX-1.2008.
	(strptime): Convert into wrapper function to provide era_info and
	alt_digits pointers and call __strptime.
	(conv_num): Take additional alt_digits_t parameter and if it's not
	NULL, call find_alt_digits to convert.