doc: generate man pages from doxygen

Create client-side and server-side man pages from doxygen. The doxygen
config options are virtually the same as for the XML output, but we do pass
in the specific options via stdin.

WL_EXPORT is predefined to the empty string, it makes the man page look
confusing and provides no value here anyway. This applies for both xml and
man output.

JAVADOC_AUTOBRIEF is disabled for man pages, the formatting in the resulting
man page is IMO hard to read.

Most of the server man pages are virtually empty, there's just not enough
documentation in the source files.

Interesting issue: the usage of @code in the protocol to reference the
parameter breaks the expansion of WL_EXPORT, thus leaving us with WL_EXPORT
in all the man pages.
Presumably this is an issue with doxygen interpreting this as a @code
command, but I already wasted enough time narrowing this down.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>