minijail: Add a way to allow arbitrary fd redirects

This change allows for the parent to redirect arbtitrary file
descriptors into the child, in a way that works even when the
minijail_close_open_fds() is used.

This can be used to pass in additional pipes to the jailed process.

Bug: 63904978
Test: make tests
Change-Id: Ia47eec575c92a08eb5380cc15dc4561572a209b3
3 files changed