Previously linking to bareossql also required linking to
bareoscats-postgresql which was not nice and error prone.
This patch hides bareoscats-postgresql as a public requirement for
bareossql making linking implicit and easy.

PostgreSQL is the only option, no need to configure that anymore.

previously you needed to configure the catalog database type for bscan
and dbcheck with an option. This is now set to PostgreSQL and the option
is ignored for backwards compatibility.

This patch links libbareoscats-postgresql with libbareos, so even the
windows linker will find all function entrypoints.

bareoscats was removed, because only bareoscats-postgresql was left and
the need for the stub backend is gone.
However, bareoscats-postgresql depends on bareossql and vice versa, so
that these modules would have to be merged into one. This patch
re-introduces bareoscats so we can avoid the dependency cycle and
postpone the merge of the two modules.

This patch removes all mentionings of MySQL and SQLite catalogs from the
documentation, leaving PostgreSQL as the only option.