SIP display-name needed to be empty for Avaya IP500

In order to address a compatability issue with certain features on certain devices
which rely on display name content to change behavior, initreqprep in chan_sip.c
has been changed to no longer substitute cid_number into the display name when
cid_name isn't present.  Instead, it will send no display name in that case.

(closes issue ASTERISK-16198)
Reported by: Walter Doekes

Review: https://reviewboard.asterisk.org/r/1341/




git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@331371 65c4cc65-6c06-0410-ace0-fbb531ad65f3