gcc_release (usage): Document that -s now takes symbolic name and branch of the snapshot as parameters.

	* gcc_release (usage): Document that -s now takes symbolic name
	and branch of the snapshot as parameters.
	Implement this as part of command-line processing.
	(BRANCH): Remove default initialization for snapshots.
	(CVSBRANCH): Ditto.
	Do not lay down a CVS tag if generating a snapshot from mainline.

From-SVN: r69708