userapi: message versioning

[Peter da Silva]

Matt: I've been thinking about what you said about versioning, and I 
think that there needs to be a convention that no message size or format 
should be changed without a change to the message: the kernel should 
always accept an older format message or reject it (EINVAL?).

Whether this is handled by updating the system call number or by having 
a version number field in the message doesn't really matter: a version 
field would make for an easier userland trap for emulating older APIs, 
changing the syscall number would make for a more efficient trap in the 
kernel.