os-posix: Expand setrlimit() syscall compatibility

os-posix: Expand setrlimit() syscall compatibility

Darwin uses a subtly different version of the setrlimit() syscall as
described in the COMPATIBILITY section of the macOS man page. The value
of the rlim_cur member has been adjusted accordingly for Darwin-based
systems.

Signed-off-by: Trent Huber <trentmhuber@gmail.com>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>