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.
the process is *NOT* allowed to exit after a srvrelease() as
it still holds a reference (srv->rref) preventing the srv
from beging freed/ended (listensrv) before srvacquire().
