RxRPC: Handle VERSION Rx protocol packets

Handle VERSION Rx protocol packets.  We should respond to a VERSION packet
with a string indicating the Rx version.  This is a maximum of 64 characters
and is padded out to 65 chars with NUL bytes.

Note that other AFS clients use the version request as a NAT keepalive so we
need to handle it rather than returning an abort.

The standard formulation seems to be:

	<project> <version> built <yyyy>-<mm>-<dd>

for example:

	" OpenAFS 1.6.2 built  2013-05-07 "

(note the three extra spaces) as obtained with:

	rxdebug grand.mit.edu -version

from the openafs package.

Signed-off-by: David Howells <dhowells@redhat.com>