The filter implementation is now fully hidden. Also converted it to a C++
struct allocated with new and stored the filters in a global list instead
of embedding the list in the object itself.
The internal header directory conflicted with in-source builds causing a
build failure. This is fixed by renaming the internal header directory to
something other than maxscale.
The renaming pointed out a few problems in a couple of source files that
appeared to include internal headers when the headers were in fact public
headers.
Fixed maxctrl in-source builds by making the copying of the sources
optional.
