Compare what's read from files against proper byte literals. Neither of these

Compare what's read from files against proper byte literals.  Neither of these
modules have unittests AFAICT, and I'm not improving things here, but these
changes make a bunch of email package tests succeed, and command line testing
against real files seems to make things better.

Added an __main__ section to imghdr.py so that it can be run from the command
line similarly to sndhdr.py.  Someone else can figure out a better -m way of
doing this.

In sndhdr.py do a couple of very minor cleanups, and use a with statement to
not wait for gc to close the file.