netfilter: nf_conntrack_sip: Dynamically enable/disable SIP ALG.

Some SIP clients behind NAT require ALG to be turned off.
Allows easy way to do SIP ALG enable/disable from userspace.
Avoids SIP ALG module loading/unloading and saves memory.

Change-Id: I7fc38b8c334838c470993424fdb7ea20e5aa60fe
Signed-off-by: Tyler Wear <twear@codeaurora.org>