r11473: Based on work by Jelmer, implement the [async] flag for rpc requests. If...

r11473: Based on work by Jelmer, implement the [async] flag for rpc requests. If it's
not there (it's not yet on *any* call... :-)), the rpc client strictly
sequences calls to an rpc pipe. Might need some more work on the exact
sequencing semantics when a pipe with both sync and async calls is actually
deployed, but I want it in for winbind simplification.

Volker