tahoe cp: overhaul target assignment, update tests

tahoe cp: overhaul target assignment, update tests

This substantially changes the internals of "tahoe cp", to behave in
accordance with the scheme developed in ticket:2329. test_cli_cp.py got
a large new test to exercise all the various combinations. This also
changes the set of error messages that "tahoe cp" can produce.

This modifies try_copy(), inserts a new implementation of
copy_things_to_directory() (and supporting methods), and fixes a few
bugs elsewhere.

fixes ticket:2329