init: Introduce class InterprocessFifo

Prepare for introducing a second interprocess communication channel by
introducing the class InterprocessFifo. Stop using std::unique_ptr<> for
holding the pipe file descriptors. Handle EOF consistently.

Bug: 213617178
Change-Id: Ic0cf18d3d8ea61b8ee17e64de8a9df2736e26728
Signed-off-by: Bart Van Assche <bvanassche@google.com>