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.
If the executed subprocess prefixes its output with either `error:`,
`warning:` or `info:`, the message will be logged on the appropriate
level. If no prefix is provided, the message is logged on the notice
level.
When the subprocess outputs a line, the message should be logged
immediately. This allows automated timestamps for the output of the
executed subprocess.
