Generate only real man pages with doxygen (MAN_LINKS=NO)

Generate only real man pages with doxygen (MAN_LINKS=NO)

WIth MAN_LINKS=YES doxygen will generate ton's of man pages for each
field, function and so on, but it will not contain anything useful
except for link to another man page (header).

So to avoid man pages clashing, disable MAN_LINKS, and this will
generate man pages only for explicitly documented parts.

But even after this we have pretty non-unique man pages names:

    $ find install/ -name '*.3'
    install/share/man/man3/util.h.3
    ...

Looks like we should add prefix for each of them...
But let's see if there will be a problem.

Fixes: #1097