Ask dispatch whether it requires serial mode.

	* retry.cc (gtm_thread::decide_begin_dispatch): Ask dispatch whether
	it requires serial mode instead of assuming that for certain
	dispatchs.
	* dispatch.h (abi_dispatch::requires_serial): New.
	(abi_dispatch::abi_dispatch): Adapt.
	* method-gl.cc (gl_wt_dispatch::gl_wt_dispatch): Adapt.
	* method-ml.cc (ml_wt_dispatch::ml_wt_dispatch): Same.
	* method-serial.cc (serialirr_dispatch::serialirr_dispatch,
	serial_dispatch::serial_dispatch,
	serialirr_onwrite_dispatch::serialirr_onwrite_dispatch): Same.

From-SVN: r192777