This came up in the comp.mail.mutt newsgroup where a user wasn't
satisfied with the SHORT_MAX seconds ~9 hours limit on passphrase
timeouts.

For the first time made it necessary for the options parser to be
able to parse numbers as long values. Also, introduced
mutt_add_timeout() to detect possible overflow before adding a
timeout to a time_t value and truncate to TIME_T_MAX instead.