styx.Topen.Flag does not perserve os.O_APPEND flag

[majiru]

When mode is signaled to include os.O_APPEND, the flag is lost when received by a Serve9P function.

Example output with a client opening with os.O_APPEND, the log output is from a
log.Printf("%q\n", t.Flag) with t being an incoming styx.Topen

→ 043 Topen fid=1496 mode=01
2019/05/06 17:46:31 '\x00'

[majiru]

Reading more, it seems that O_APPEND isn't an open mode with 9p.

I had assumed it was based on the mention here https://github.com/droyo/styx/blob/master/request.go#L108.