swift234

[libdispatch] Post-beta API changes and bug fixes

    * Fix DispatchSourceSignal initialisation such that it no longer
    registers for the wrong source type.

    * Remove (group:) option from DispatchWorkItem, introduce group
    options to `.async` methods that accept DispatchWorkItem.

    * Rename `DispatchSourceType` to `DispatchSourceProtocol`

    * Rework DispatchQueue attributes and flags into a less confusing
    approach.

    * Fixes:

	SR-1817, SR-1771, SR-1770, SR-1769

	<rdar://problem/26725156> <rdar://problem/26873917>
	<rdar://problem/26918843> <rdar://problem/26810149>
	<rdar://problem/27117023> <rdar://problem/27121422>
	<rdar://problem/27236887> <rdar://problem/27337555>