[proposal] reserved identifiers

[Richard Godard <richard.godard@tecfa.unige.ch>]

I can't find in the specs that mention something about reserved
identifier/keywords in message keywords, for example in the case of cords:

#$#mcp-cord 3487 _id: I12345 _message: <enclosed message name> <enclosed
message keywords>

The usage of cords is not completely transparent to the user since if the
enclosed message keywords contain some keywords which are either _id or
_message then the whole message is mangled.

I would like to propose a minimal extension to the specs:

Keyword of the form _<pakage-name>-<ident> are reserved for the sole usage
of <pagkage-name>.

Richard