9p: trans_fd, read rework to use p9_parse_header

Most of the changes here are no-op and just renaming to use a
fcall struct, needed for p9_parse_header

It fixes the unaligned memory access to read the tag and defers to
common functions for part of the protocol knowledge (although header
length is still hard-coded...)

Reported-By: Rob Landley <rob@landley.net>
Signed-Off-By: Dominique Martinet <dominique.martinet@cea.fr>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>