Expand and expose ctrl connection allocation

Add function for allocating CTRL connection to public headers and
replace call to previous static function with it. Add doxygen docs for
this function.

It's useful if we need to allocate ctrl connection but don't need to
bind to any interfaces: when we act as ctrl client.

Related: OS#1615
Change-Id: I522ed809cbebfd3d7dd08b4ed9137b39ff192e32
2 files changed