Atomically update the global gMsgId in libnetgraph.

Otherwise concurrently running threads may inadvertently use the same
token for different messages.

Preserve the behaviour of disallowing negative message tokens, but allow
a message token value of zero since this simplifies the code a bit and
tokens are documented to be non-negative.

PR: 234442
Reported and tested by: eugen
MFC after: 1 month
Sponsored by: The FreeBSD Foundation