Adding dialplan function IFMODULE, so you can create dialplans that handle

various PBX installations and checks if a module is loaded before using
it.
	example IFMODULE(chan_sip3.so)

issue #6671 in the bug tracker, finally gone. Thanks to mithraen for keeping
it updated.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@46513 65c4cc65-6c06-0410-ace0-fbb531ad65f3