Make POSIX fully responsible for assigning a Target to a File.

This eliminates the awkward need to initialize a File with a NULL Target. This
is effectively what still happens, but it is behind-the-scenes now.
Move stdout handling into the POSIX emulation class via a File.
Move stderr handling into the POSIX emulation class via a File.

This generalizes the fopen-family of wrappers a bit. This also cleans up the
logging infrastructure.
Add ability to register filenames in PepperPOSIX::POSIX.

This removes the final specialization in fopen-family of calls, specifically
for "/dev/urandom". All wrappers are now general.