rpc: Allow xdr_buf_subsegment to operate in-place

Allow

	xdr_buf_subsegment(&buf, &buf, base, len)

to modify an xdr_buf in-place.

Also, none of the callers need the iov_base of head or tail to be zeroed
out.

Also add documentation.

(As it turns out, I'm not really using this new guarantee, but it seems
a simple way to make this function a bit more robust.)

Signed-off-by: J. Bruce Fields <bfields@redhat.com>