env -C lacks fchdir support

[Eric Blake]

There are cases where you cannot directly change directory to a given 
location via a path name, but where you can hold an fd to that directory 
and where fchdir() will work. Since we've already added 'env -C' as an 
extension for chdir, should we add a counterpart extension that permits 
fchdir to an inherited fd, even for systems that lack /proc/NNN/fd/MMM?

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
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