Build: Fix construction of manual pages.

Build: Fix construction of manual pages.

Firstly, there was a bug in vars.am: the suffix rule used to construct
manpages was broken because suffix rules aren't allowed to have
dependencies of their own.  So purge defs.man.in (we now just have
defs.man) and confsubst the entire manpage each time.

Secondly, in preparation for new manpages for services, consolidate the
summary-building machinery into vars.am.  The server makefile no longer
needs a special case for tripe-admin.8.

To keep things tidy, defs.man and make-summary have been stashed in
common.  This seems as good a place as any.