Add separate configuration options for subscription and registration minexpiry and maxexpiry.

This offers more fine-grained control over how long subscriptions last without negatively
affecting the expiration range for registrations.

Uploaded by:
	Guenther Kelleter(license #6372)

Review: https://reviewboard.asterisk.org/r/2051



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@370386 65c4cc65-6c06-0410-ace0-fbb531ad65f3