Message queues are basically just files and in most cases can be handled
like normal files. So it was a mistake to set the "on-disk-device" flag
for them to fix the mq_unlink() problem reported in
https://cygwin.com/pipermail/cygwin/2025-January/257119.html
Rather, given that unlink() just checks if the object to be deleted
has an on-disk representation, make sure message queues are added to
the path_conv::isondisk() predicate.
This also reverts commit d870655f570f25393dcefbaf0b1dc807f277749c.
Fixes: d870655f570f ("Cygwin: path_conv: set on-disk-device flag for message queue files")
Reported-by: Christian Franke <Christian.Franke@t-online.de>
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>
b940faa144