sip_to_pjsip: Fix missing cases

Adds the "auto" case which is valid with
both chan_sip dtmfmode and chan_pjsip's
dtmf_mode, adds subscribecontext to
subscribe_context conversion, and accounts
for cipher = ALL being invalid.

ASTERISK-29459

Change-Id: Ie27d6606efad3591038000e5f3c34fa94730f6f2