Reopen whitelisted zygote file descriptors after a fork.

We don't want these descriptors to be shared post-fork, so we'll
have to close and reopen them when the zygote forks. The set of
open descriptors is checked against a whitelist and it is a fatal
error if a non whitelisted FD is opened. It is also a fatal error
if anything other than a regular file / character device or socket
is opened at the time of forking.

This work is done in two stages :
- An initial list of FDs is constructed and cached prior to the
  first zygote fork.

- On each subsequent fork, we check whether the list of open FDs
  has changed. We are currently tolerant of changes, but in the
  longer term, it should be a fatal error if the set of open file
  descriptors in the zygote changes.

- Post fork, we traverse the list of open descriptors and reopen
  them if necessary.

bug: 30963384
Change-Id: Icfd45c96c660491b554d146a9d70d97dbcc712bc