with the latest changes to shr(3), we can use ORCLOSE on
the control file to get the mount in the share automatically
removed when the server exits or something goes wrong during
postsharesrv().
do not expose postfd() and sharefd() functions. they where
undocumented and leak the control file descriptors.
it is unclear how Srv.nopipe flag should work inside
postmountserv(). if a server wants to serve on stdio
descriptors, he can just call srv() after initializing
Srv.infd and Srv.outfd.
The Srv.leavefdsopen hack can be removed now that acme
win has been fixed.
listensrv() used to override Srv.end() with its own handler
to free the malloc'd Srv structure and close the fd. this
makes it impossible to register your own cleanup handler.
instead, we introduce the private Srv.free() handler that
is used by listensrv to register its cleanup code. Srv.free()
is called once all the srv procs have been exited and all
requests on that srv have been responded to while Srv.end()
is called once all the procs exited the srv loop regardless
of the requests still being in flight.
in multithreaded programs, we have to wait until all outstanding
requests have been responded before closing down the srv.
dont make write errors sysfatal(), only print them. in case if
listensrv() is used we dont want to exit the process in respond()
called by some worker thread.
make sure Tversion is only handled when there are no outstanding
requests and make sure message size is sane.
