Makefile: release as tar.xz and don't include HTML docs by default.

Since this is mainly a server utility, the HTML page is a bit
redundant because we already include the man page from which it is
built. Anyone is capable of running man2html to turn a man page into a
web page if they so desire.

We also change the release tarball to "tar.xz" format with lzma
compression, because that's what all the cool kids are doing.