chan_pjsip: Make logger togglable without loading/unloading

chan_pjsip: Make logger togglable without loading/unloading

This patch makes the res_pjsip_logger do a few things... First, it
will be built and installed by default now, so end users won't need
to enable it in menuselect. Second, while it is loaded, it no longer
will immediately issue log messages. Upon loading, it is in the
disabled state and must be turned on with the new CLI command. The
CLI command 'pjsip set logger <on/off/host> has been added and can be
used to do the following:
pjsip set logger on:
    Enables logger for all PJSIP traffic
pjsip set logger off:
    Disables logger for all PJSIP traffic
pjsip set logger host <host>:
    Enables logger for the specific host

Review: https://reviewboard.asterisk.org/r/2900/
........

Merged revisions 400542 from http://svn.asterisk.org/svn/asterisk/branches/12

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@400543 65c4cc65-6c06-0410-ace0-fbb531ad65f3