sms: Introduce VTY-configurable minimum SMS validity period

This is meant as a safeguard against users or user equipment which
doesn't set a reasonable validity period.  Using this setting, the
SMSC administrator can set a minimum SMS validity period. Any SMS
submitted with lower validity period will be extended to that minimum.

Change-Id: I192528a6f9059d158fa12876a247d61bd7edaec8
Related: OS#5567
6 files changed