[PATCH] sms: Proper decoding and storage of SMS validity period.

- Use time_t instead of abstract minutes to store validity time to make sure we support all possible values. - Return UNIX time from it to support both relative and absolute expire times. - Pass a current time to gsm340_validity_period() to compute the absolute expire time if a relative time is specified. What time is "current" is to b - We also introduce SMS_DEFAULT_VALIDITY_PERIOD macros to reduce number of magic values.

PS This is a first patch of a series of patches which fix SMS validity time decoding. I'm cleaning the patch set and want to see if this patch is fine, as other patches rely on it.