Bug 28460 - Refactored dbus configuration access.

Bug 28460 - Refactored dbus configuration access.

Libdbus uses several config variables. On unix these settings are read from
environment variables by using _dbus_getenv.

On other platforms like wince there are no environment variables available and
_dbus_getenv needs an emulation for those plattforms (see
dbus/dbus-sysdeps-wince-glue.c)

To cleanup this emulation the appended patch adds a config api by adding
_dbus_config_... functions.

Also having all client config related functions listed in one header file
provides a good overview about which config attributes  are available.

The default implementation retrieves the config values from environment
variables. For other os this could be easily extended or replaced by.