rtp: Ensure defaults are set without rtp.conf.

While building up a new install to test chan_motif, I ran into a failure
due to icesupport being disabled.  This was due to me not having an
rtp.conf.  It was intended in the code for it to be enabled by default,
but it was only applied if rtp.conf existed.

This patch updates res_rtp_asterisk to be consistent in how it handles
defaults.  A few options didn't have their default values set globally,
including icesupport.  They are now set and icesupport is enabled by
default, even if you do not have an rtp.conf.
........

Merged revisions 371425 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@371428 65c4cc65-6c06-0410-ace0-fbb531ad65f3