Fix 'arcyd rm-repo' name arg help string

Make the help string relevant to the arg, this appears to be a
copy-paste error.

Test Plan:
Proof-read generated doc
Doc, fix typo 'the a' -> 'a'

Fix a silly typo in a docstring.
'arcyd rm-repo' commits the change, as 'add-repo'

Make 'arcyd rm-repo' symmetrical with 'arcyd add-repo' in that it
commits the change to the filesystem to the underlying config file git
repository on behalf of the adminstrator.

Add appropriate methods to abdt_fs.Accessor for removing configs.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/test_shell.sh
    $ cd ../arcyd
    $ arcyd stop
    $ arcyd rm-repo localhost_fs1_origin
    $ git show
    .. HEAD commit has removal as expected ..
Fix 'arcyd rm-repo' to tolerate missing files

When removing repos, it's possible that some of the files associated
with the repository may be missing because they were never cloned or the
repo has never been processed.

Print warnings in the event that these files are missing and continue.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/test_shell.sh
    $ cd ../arcyd2/
    $ arcyd rm-repo localhost_fs2_origin2
    .. warn about missing .try and .ok files ..
    $ git show
    .. commit removes repo config as expected ..
Refactor, 'arcyd list-repos' to re-use utility fn

Simplify 'arcyd list-repos' by re-using the
abdi_repoargs.parse_config_file_list() function instead of the local
custom _iter_repo_args().

Test Plan
$ ./precommit.sh
$ ./testbed/arcyd/test_shell.sh
  $ cd ../arcyd
  $ arcyd list-repos
  .. looks right ..
  $ arcyd list-repos --only-formatted-repo-urls
  .. looks right ..
Test, exercise arcyd list-repos, fsck, rm-repo

Add some simple smoke tests for these commands to the exercise_arcyd.sh
script.

Test Plan:
$ ./testbed/arcyd/exercise_arcyd.sh
Check it exits when expected and that 'git show' output is as expected.
Allow urls with 'arcyd rm-repo' for ease of use

Make it easier to remove repo configs by adding a flag to lookup urls.

Test Plan
$ ./precommit.sh
$ ./testbed/arcyd/test_shell.sh
    $ cd ../arcyd
    $ arcyd stop
    $ arcyd rm-repo --lookup-url ../origin
    $ git show
    .. looks right ..