build: Rename clean targets.

The documentation of gnu make recommends to have targets called
mostlyclean, clean, distclean and maintainer-clean while we have clean,
clean2, distclean and maintainer clean.

This patch adds mostlyclean and removes clean2. The former target
removes the executables and all object files, clean additionally
removes the whole build directory and distclean removes, on top of
that, all files created by autoconf. Finally, maintainer-clean removes
the tarball and the files created by doxygen and global.