t/: Make mock_plugin a "check" library.

This way, it's only built on 'make check' (aka 'make test').
Makefile: For now, link libsysdb sources directly into sysdbd.

Libtool's convenience libraries don't work with programs. When linking sysdbd
previously, none of the libsysdb objects only used by plugins would have been
linked in. That is, the respective libraries could not be loaded when running
the daemon.

We'll keep the library around, though, since it's used for the unit tests.

This is just a hack but it'll go away after the API is stable and libsysdb is
turned back into a "real" library. That'll happen before the 1.0 release.
client: Added sdb_client_shutdown().
sysdb: Wait for remaining data after the mainloop terminates.
plugin: Ignore internal loggers when checking for existing loggers.

An internal logger may currently be registered by the frontend which uses it
to send log messages to a client. Usually, these internal loggers will not
actually log stuff. Thus, we'll have to ignore them when deciding about
fallbacks.
store: Removed obsolete TODO note.
store: Renamed a store object's 'children' to 'services'.

Hierarchical data will be represented using some object relationship
information rather than a weird tree construct below each host. That's going
to be more flexible.
store: Simplified object update based on having hosts on the toplevel only.
store: Simplified object lookup during update.

Again, based on having hosts on the toplevel only. This greatly simplifies the
overall structure. After all, this is the underlying storage layer only. More
complex relationships between objects will be handled on a different layer.
Merged branch 'master' of git://git.tokkee.org/sysdb.