add srv

srv makes a connection to the argument and drops a 9p server in '#s'

Doesn't work at present, but I'm working on it.

usage:
srv 'tcp!10.0.2.1!20000' (e.g.)
mount '#s/tcp!10.0.2.1!20000' /mnt

Signed-off-by: Ronald G. Minnich <rminnich@google.com>