Give every Makefile.nmake file a "distclean" rule, and have "distclean"

Give every Makefile.nmake file a "distclean" rule, and have "distclean"
recurse into subdirectories doing "nmake -f Makefile.nmake distclean".

Have "nmake -f Makefile.nmake clean" not remove stuff that "make clean"
doesn't remove (such as Flex/Bison output and config.h files) - and have
"nmake -f Makefile.nmake distclean" remove stuff that "make distclean"
removes, including "tethereal-tap-register.c" and
"ethereal-tap-register.c".

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@8672 f5534014-38df-0310-8fa8-9805f1628bb7