smpp: Parse and use SMPP-provided validity period

Before this patch, we always ignored any SMPP-provided validity period
and used '0' which is now, and means it expires immediately.

As SMPP allows for validity_period of NULL, use 7 days as SMSC default
in such situations.

Change-Id: Iad9f2697f045ed3bc0eb74c3a9730861f82e6c48
Closes: OS#5567
diff --git a/src/libmsc/smpp_smsc.h b/src/libmsc/smpp_smsc.h
index 4c0d701..257589e 100644
--- a/src/libmsc/smpp_smsc.h
+++ b/src/libmsc/smpp_smsc.h
@@ -158,6 +158,7 @@
 
 int smpp_determine_scheme(uint8_t dcs, uint8_t *data_coding, int *mode);
 
+time_t smpp_parse_time_format(const char *vp, time_t *t_now);
 
 
 struct gsm_sms;