Use and watch SYS_ugetrlimit rather than SYS_getrlimit

For 32-bit, SYS_getrlimit uses an old rlimit struct with smaller fields.
We need to use SYS_ugetrlimit and monitor the app's use of the same for
-steal_fds.

Augments the file_io test of -steal_fds to test getrlimit, both the old and
the new.
Adds -DUNIX to the build for tests which was missing.

Review-URL: https://codereview.appspot.com/188970043