init: clean up file / socket descriptor creation

clang-tidy hinted that some of this code wasn't right.  Looking
deeper, there is really not much related to file and socket
descriptors, except that they're published in similar ways to the
environment.  All of the abstraction into a 'Descriptor' class takes
us further away from specifying what we really mean.

This removes that abstraction, adds stricter checks and better errors
for parsing init scripts, reports sockets and files that are unable to
be acquired before exec, and updates the README.md for the passcred
option.

Test: build, logd (uses files and sockets) works
Change-Id: I59e611e95c85bdbefa779ef69b32b9dd4ee203e2