Major rewrite of RCS backends. RCS now represented as a class.

Major rewrite of RCS backends.  RCS now represented as a class.

Lots of changes and just one commit.  This started with bug
dac91856-cb6a-4f69-8c03-38ff0b29aab2, when I noticed that new bugs
were not being added appropriately with the Git backend.  I'd been
working with Git trouble before with bug
0cad2ac6-76ef-4a88-abdf-b2e02de76f5c, and decided things would be
better off if I just scrapped the current RCS architecture and went to
a more object oriented setup.  So I did.  It's not clear how to add
support for an RCS backend:
 * Create a new module that
   - defines an inheritor of rsc.RCS, overriding the _rcs_*() methods
   - provide a new() function for instantizating the new class
   - defines an inheritor of rcs.RCStestCase, overiding the Class attribute
   - defines 'suite' a unittest.TestSuite testing the module
 * Add your new module to the rest in rcs._get_matching_rcs()
 * Add your new module to the rest in libbe/tests.py
Although I'm not sure libbe/tests.py is still usefull.

The new framework clears out a bunch of hackery that used to be
involved with supporting becommands/diff.py.  There's still room for
progress though.  While implementing the new verision, I moved the
testing framework over from doctest to a doctest/unittest combination.
Longer tests that don't demonstrate a function's usage should be moved
to unittests at the end of the module, since unittest has better
support for setup/teardown, etc.

The new framework also revealed some underimplented backends, most
notably arch.  These backends have now been fixed.

I also tweaked the test_usage.sh script to run through all the backends
if it is called with no arguments.

The fix for the dac bug turned out to be an unflushed file write :p.