rpc: use count of pipe openers to wait for first open

Introduce a global variable pipe_version which will eventually be used
to keep track of which version of the upcall gssd is using.

For now, though, it only keeps track of whether any pipe is open or not;
it is negative if not, zero if one is opened.  We use this to wait for
the first gssd to open a pipe.

(Minor digression: note this waits only for the very first open of any
pipe, not for the first open of a pipe for a given auth; thus we still
need the RPC_PIPE_WAIT_FOR_OPEN behavior to wait for gssd to open new
pipes that pop up on subsequent mounts.)

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>