add `cmd` (command output expression).
STDERR's file descriptor is 2.

closes #1
use array parameter on IO.open.
suppress a warning and fix a possible segv.
remove duplicated white spaces.
fix RSTRING_PTR usage.
IO.sysopen raises an exception when open(2) fails.
test for #6.
Try GC when Too many open files (revert a part of 1e8097a).