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.