os-spec/linux: additional device nodes

[lucab]

Restarting the previous discussion at #127, it looks like we may need new entries in the set of provided /dev paths.

In particular, it may be useful to have the runtime provide the following paths:

• /dev/fd/
• /dev/stdin
• /dev/stdout
• /dev/stderr

Rationale for this is:

1. OCI makes those mandatory as well
2. docker2aci already add them to converted images
3. generic ACI images typically don't embed them
4. simple things like bash process substitution break without them

[alban]

But simple bash scripts like echo foo > /dev/fd/1 will not work in rkt because rkt uses journald (unless rkt is run with --interactive) and /dev/self/fd/... are connected to the journald unix socket.

So I am trying to discourage opening /dev/fd/* directly:
https://github.com/kubernetes/kubernetes/pull/23381/files
rkt/rkt#2300 (comment)

[jonboulle]

should we nevertheless add the others..?