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 -- - Daniel Willmann <dwillmann(a)sysmocom.de> http://www.sysmocom.de/ ======================================================================= * sysmocom - systems for mobile communications GmbH * Schivelbeiner Str. 5 * 10439 Berlin, Germany * Sitz / Registered office: Berlin, HRB 134158 B * Geschaeftsfuehrer / Managing Directors: Holger Freyther, Harald Welte