[wmii] libixp/fs.c: wstat?

Hi,

it seems wstat was implemented somewhen ago in libixp. As far as I could read
on the web, wstat is a 9p-call to change the fields of stat for a specific
file. It seems to be used to truncate files (on WSL 2.6.28.2 with 9pfs).

However, I've noticed three problems:
1) libixp does not check whether wstat is actually provided by the server. The
   attached pull request fixes it. I'm not so sure if this is done right, but at
   most there is no less segfault when trying to write "toggle fullscreen" to
   /client/sel/ctl (I know, it should be "Fullscreen toggle", but use it as a
   test case).

2) wmii does not implement wstat, ergo writing to meta files (/ctl) does not
   work on my thin client (while writing to /rbar works), because echo cannot
   truncate the file.

3) When writing "Fullscreen toggle" to /client/sel/ctl, something (probably
   9pfs) produces a hard-lockup (sound freezes, Wayland freezes, nothing possible
   no less, just reboot). This may be related to 2). I'm not sure how to
   further debug this.

Best regards,
Michael