Portable Message Queues

[Guy Harris]

> Is O_EXCL portable?  I know System V has it, but possibly not BSD.

4.2BSD and subsequent BSD releases have all the O_* flags that S5R2 had,
including O_EXCL.  (O_NDELAY has some differences in its behavior, however.)
No current BSD release has the O_EXCL flag that first appeared in a release
after S5R2 (there was a bit that did some of O_EXCL in earlier releases, but it
was for use inside the kernel only, and was, as I remember missing some pieces
that were necessary to make it useful; I guess they put in the pieces and
advertised the flag).  It's probably not hard to add to BSD.