docs: Honour --{en, dis}able-xend when building docs

docs: Honour --{en, dis}able-xend when building docs

If a user has specified --disable-xend, they wont want the manpages either.

Propagating this parameters requires reorganising the way in which the
makefile chooses which documents to build.

There is now a split of {MAN1,MAN5,MARKDOWN,TXT}SRC-y to select which
documentation to build, which is separate from the patsubst section which
generates appropriate paths to trigger the later rules.

The manpages are quite easy to split between xend, xl and xenstore, and have
been.  Items from misc/ are much harder and been left.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
[ ijc -- reran autogen.sh as requested. ]