The same mechanism that is used for modules can be used for the
configuration of the core objects. This removes the need for the redundant
code that validates various values that is already present in the code
that modules use.
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.
