New XML-based man page build

Following up to the recent discussion that our man page build is weird,
ancient, and only works with patched tools, I have implemented support
for using a newer toolchain based on docbook2x(.sf.net).  You can try it
out by installing docbook2x and running "make manx".

Right now, I can only verify that it works on Debian.  Installing this
package from sources might be pretty complicated.

The build works nicely and quickly, and if we fix up a few small
formatting artifacts and sort out the installation issues, we could
switch to using this as our standard build method.