backend for config file loader. Doesn't work with full OOM test yet.

* configure.in: change error when selecting libxml into a warning.

* test/data/invalid-config-files: add two non-well-formed XML files.

* glib/Makefile.am: libdbus_gtool always uses expat, not libxml.

* dbus/dbus-transport-unix.c (unix_handle_watch): do not disconnect in
case of DBUS_WATCH_HANGUP, several do_reading() may be necessary to
read all the buffer. (bug #894)

* bus/activation.c (bus_activation_activate_service): fix a potential
assertion failure (bug #896). Small optimization in the case of
auto-activation messages.

* dbus/dbus-message.c (verify_test_message, _dbus_message_test): add
test case for byte-through-vararg bug (#901). patch by Kimmo
Hämäläinen.