FreeBSD /dev/fd/ again

[Bart Schaefer <schaefer@xxxxxxxxxxxxxxxx>]

In thinking about this, I've decided that

(a) it was a really silly decision on the part of FreeBSD to make this a
special file system that might (not) be mounted, because it means that
programs can't rely on any particular behavior, and

(b) as a consequence of this, zsh probably has to make its decision to
use /dev/fd/XX (or not) at run-time, rather than at compile time.

Consider what happens if you compile zsh with this filesystem mounted and
then unmount it.

Any of you folks running FreeBSD ... is there a sysconf() or similar
call that can be made to determine whether this feature is available?