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
6 files changed