Add back leading operator enforcement.
See script directory for method. The script to run in the top level MaxScale directory is called maxscale-uncrustify.sh, which uses another script, list-src, from the same directory (so you need to set your PATH). The uncrustify version was 0.66.
Take almost all warnings into use except `format-overflow` and `unused-function`. format-overflow needs to be handled in a separate commit.
GCC 8 enables -Werror=return-type by default which combined with -Werror causes the build to fail.
