Implement gsm340_{gen_,}scts() without using tm_gmtoff

Hello,

this patch series depends on two patches from Alexander (once they sign handling is fixed):

sms: Fix gsm340_scts() to correctly decode absolute valid times. sms: Fix support of negative timezone offsets in gsm340_gen_scts().

The first patch adds a test case to check that gsm340_scts is the inverse of gsm340_gen_scts.

The next patches add a helper function to calculate the gmt offset and use those in the *_scts functions.

The helper function is somewhat ugly, but it is the only portable way I could find to get the GMT offset. Ideas welcome

Regards, Daniel