Don't install rpcapd if we weren't built with --enable-remote.

And do uninstall rpcapd with "make uninstall".  (We can uninstall it
unconditionally, as it uses -f with rm, so that it doesn't fail if the
thing being uninstalled isn't installed.)

Also, have "make clean" and "make distclean" clean up the rpcap
subdirectory.