Restore flushing of output for revoke(2) again. Document revoke()'s

Restore flushing of output for revoke(2) again.  Document revoke()'s
intended behaviour in its man page.  Simplify tty_drain() to match.
Don't call ttydevsw methods in tty_flush() if the device is gone
since we now sometimes call it then.

The flushing was supposed to be implemented by passing the FNONBLOCK
flag to VOP_CLOSE() for revoke().  The tty driver is one of the few
that can block in close and was one of the fewer that knew about this.

This almost worked in FreeBSD-1 and similarly in Net/2.  These
versions only almost worked because there was and is considerable
confusion between IO_NDELAY and FNONBLOCK (aka O_NONBLOCK).  IO_NDELAY
is only valid for VOP_READ() and VOP_WRITE().  For other VOPs it has
the same value as O_SHLOCK.  But since vfs_subr.c and tty.c
consistently used the wrong flag and the O_SHLOCK flag is rarely set,
this mostly worked.  It also gave the feature than applications could
get the non-blocking close by abusing O_SHLOCK.

This was first broken then fixed in 1995.  I changed only the tty
driver to use FNONBLOCK, as a hack to get non-blocking via the normal
flag FNONBLOCK for last closes.  I didn't know about revoke()'s use
of IO_NDELAY or change it to be consistent, so revoke() was broken.
Then I changed revoke() to match.

This was next broken in 1997 then fixed in 1998.  Importing Lite2 made
the flags inconsistent again by undoing the fix only in vfs_subr.c.

This was next broken in 2008 by replacing everything in tty.c and not
checking any flags in last close.  Other bugs in draining limited the
resulting unbounded waits to drain in some cases.

It is now possible to fix this better using the new FREVOKE flag.
Just restore flushing for revoke() for now.  Don't restore or undo any
hacks for ordinary last closes yet.  But remove dead code in the
1-second relative timeout (r272789).  This did extra work to extend
the buggy draining for revoke() for as long as possible.  The 1-second
timeout made this not very long by usually flushing after 1 second.

Submitted by:	bde
MFC after:	2 weeks