drainworkbuf is now gcDrain, since it drains until there's
nothing left to drain.  drainobjects is now gcDrainN because it's
the bounded equivalent to gcDrain.

The new names use the Go camel case convention because we have to
start somewhere.  The "gc" prefix is because we don't have runtime
packages yet and just "drain" is too ambiguous.

Change-Id: I88dbdf32e8ce4ce6c3b7e1f234664be9b76cb8fd
Reviewed-on: https://go-review.googlesource.com/4785Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>