Add an option to close all open file descriptors

This adds the minijail_close_open_fds() API to close all open file
descriptors (except the pipes that are internally set up to communicate
with the jailed process).

Bug: 32005517
Test: libminijail_unittest
Change-Id: Ia392f14c080716297c5766ad31af983ee6c5ead3
3 files changed