Fix in-source builds

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.

maxctrl/build.sh

@@ -6,9 +6,15 @@ then
     exit 1
 fi
 
-# Copy sources to working directory
 src=$1
-cp -r -t $PWD/maxctrl $src/maxctrl/* && cd $PWD/maxctrl
+
+if [ "$PWD" != "$src" ]
+then
+    # Copy sources to working directory
+    cp -r -t $PWD/maxctrl $src/maxctrl/*
+fi
+
+cd $PWD/maxctrl
 
 npm install
 npm install pkg@4.2.3