shared connections fixes

[Havoc Pennington]

John (J5) Palmieri wrote:
> _dbus_abort is not defined if asserts are disabled but _dbus_warn is.
> There are two ways we can deal with this.  We can print out a warning
> that DBUS_FATAL_WARNINGS only work if asserts are compiled in or we
> could pull _dbus_abort out of the DBUS_DISABLE_ASSERT which opens up a
> whole other can of worms because _dbus_print_backtrace I believe is also
> wrapped in the macro.  I assume just not aborting is fine since a user
> would rarely ever want to abort on a warning but I thought I might check
> first.

This is the patch Thiago posted. I thought it was fine to just always 
compile in dbus_abort and dbus_print_backtrace as in his patch. It's not 
like these are large functions.

Havoc