Take Daemon stdin/stdout/stderr args as file objects

Take Daemon stdin/stdout/stderr args as file objects

Previously Daemon constructor took stdin/stdout/stderr as
paths (defaulting to '/dev/null') and opened them as regular files.
This greatly limits the type of filehandles supported (no pipes, for
example), and doesn't allow simple things like reusing existing fds.

This change switches to accepting file objects rather than strings,
and uses a sentinal value to represent the previous "open /dev/null"
default behaviour.

Change-Id: I51b36ce912194abd89ed46fad9943802f271444a