bug#15970: [Bug-tar] Crash in gettext() after fork() on Mac OS X

[Paul Eggert]

Pádraig Brady wrote:

> How about the attached instead that just uses
> a somewhat degraded but simpler error() equivalent.

That looks safe, though it could be simplified:
use inttostr instead of repeating its body,
and since !!p == !!errnum there's no need
to have those duplicate conditions.
Plus, just use STDERR_FILENO rather than
fiddling with fileno (stderr), and async_safe_error
should be _Noreturn and should unconditionally call
_exit.  And, I wouldn't bother looking at dup2's
return value, any more than we look at close's.