Per-instance devpts

[H. Peter Anvin]

Since the issue of PTY namespaces came up (and was rejected) back in 
April, I have thought a little bit about changing ptys to be tied 
directly into a devpts instance.  devpts would then be a "normal" 
filesystem, which can be mounted multiple times (or not at all).  pty's 
would then become private to a devpts instance.

This is what it would appear would have to change, and I'd like to get 
people's feeing for the user-space impact:

1. /dev/ptmx would have to change to a symlink, ptmx -> pts/ptmx.
2. Permissions on /dev/ptmx would not be persistent, and would have to
   be set via devpts mount options (unless they're 0666 root.tty, which
   would presumably be the default.)
3. The /proc/sys/kernel/pty limit would be global; a per-filesystem
   limit could be added on top or instead (presumably via a filesystem
   mount options.)

I worry #1 would have substantial user-space impact, but I don't see a 
way around it, since there would be no obvious way to associate 
/dev/ptmx with a filesystem.

	-hpa
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