This avoids an error case where an empty child name resulted in a
duplicate mkdir. It adds a precondition check to guard against empty
child names, and some test cases. It also cleans up a funny redundancy
noticed earlier (refs ticket:2329).
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
