res_pjsip_registrar: Remove unavailable contacts if exceeds max_contacts

The behavior of max_contacts and remove_existing are connected.  If
remove_existing is enabled, the soonest expiring contacts are removed.
This may occur when there is an unavailable contact.  Similarly,
when remove_existing is not enabled, registrations from good
endpoints are rejected in favor of retaining unavailable contacts.

This commit adds a new AOR option remove_unavailable, and the effect
of this setting will depend on remove_existing.  If remove_existing
is set to no, we will still remove unavailable contacts when they
exceed max_contacts, if there are any. If remove_existing is set to
yes, we will prioritize the removal of unavailable contacts before
those that are expiring soonest.

ASTERISK-29525

Change-Id: Ia2711b08f2b4d1177411b1be23e970d7fdff5784

configs/samples/pjsip.conf.sample

@@ -1109,6 +1109,16 @@
                         ; contact created by rewrite_contact that the device is
                         ; refreshing.
                         ; (default: "no")
+;remove_unavailable=no  ; If remove_existing is disabled, will allow a registration
+                        ; to succeed by removing only unavailable contacts when
+                        ; max_contacts is exceeded. This will reject a registration
+                        ; that exceeds max_contacts if no unavailable contacts are
+                        ; present to remove. If remove_existing is enabled, will
+                        ; prioritize removal of unavailable contacts before removing
+                        ; expiring soonest.  This tames the behavior of remove_existing
+                        ; to only remove an available contact if an unavailable one is
+                        ; not present.
+                        ; (default: "no")
 ;type=  ; Must be of type aor (default: "")
 ;qualify_frequency=0    ; Interval at which to qualify an AoR via OPTIONS requests.
                         ; (default: "0")