res_pjsip_session: Enable RFC3578 overlap dialing support.

Support for RFC3578 overlap dialling (i.e. 484 Response to partially matched
destinations) as currently provided by chan_sip is missing from res_pjsip.
This patch adds a new endpoint attribute (allow_overlap) [defaults to yes]
which when set to yes enables 484 responses to partial destination
matches rather than the current 404.

ASTERISK-26864

Change-Id: Iea444da3ee7c7d4f1fde1d01d138a3d7b0fe40f6

contrib/ast-db-manage/config/versions/8fce4c573e15_add_pjsip_allow_overlap.py

@@ -0,0 +1,31 @@
+"""add pjsip allow_overlap
+
+Revision ID: 8fce4c573e15
+Revises: f638dbe2eb23
+Create Date: 2017-03-21 15:14:27.612945
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '8fce4c573e15'
+down_revision = 'f638dbe2eb23'
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects.postgresql import ENUM
+
+YESNO_NAME = 'yesno_values'
+YESNO_VALUES = ['yes', 'no']
+
+def upgrade():
+    ############################# Enums ##############################
+
+    # yesno_values have already been created, so use postgres enum object
+    # type to get around "already created" issue - works okay with mysql
+    yesno_values = ENUM(*YESNO_VALUES, name=YESNO_NAME, create_type=False)
+
+    op.add_column('ps_endpoints', sa.Column('allow_overlap', yesno_values))
+
+
+def downgrade():
+    op.drop_column('ps_endpoints', 'allow_overlap')