netfilter: nf_conntrack_sip: Dynamically enable/disable SIP direct media.

Enabling SIP direct media allows flow of SIP media traffic only between
the SIP signalling end points. Disabling SIP direct media allows flow of
SIP media traffic between 2 SIP clients which interact with each other
with the help of SIP server as the intermediate entity, which is the more
common usecase. This change allows easy way to enable/disable SIP direct
media from userspace to cater to the more common usecase.

Change-Id: Iadfd7062e48c8b4e2f7af919a6062360c165344c
Signed-off-by: Tyler Wear <twear@codeaurora.org>