remove deprecated AM_MAINTAINER_MODE from configure.ac

See
www.gnu.org/software/automake/manual/html_node/maintainer_002dmode.html#maintainer_002dmode
for the rationale.  Summary:

  ... François Pinard pointed out several arguments against this
  ... macro ... Jim Meyering, the inventor of the AM_MAINTAINER_MODE
  macro was swayed by François’s arguments, and got rid of
  AM_MAINTAINER_MODE in all of his packages.
autoupdate
rm include/stamp-h.in (unneeded)
rm libnet/libnet-config.in, as libnet-config is no longer hooked into the build system
section header for header section in configure.ac
git ignore libnet/compile (generated by autotools)
use available autotools doxygen support
git ignore dox debris
more specific ignorance