phlsys_fs: add 'ensure_dir', 'write_text_file'

Make it easy to write text files and other files at arbitrary paths
without having to worry about creating the intermediate directories
along the path.

Test Plan:
$ ./precommit.sh
abdi_processargs: nicer cli argument strings
Arcyd: add 'init' command

Arcyd is hard to setup.

Make it a bit easier to get started by providing an 'init' command
which create a directory structure managed by a git repo, containing
helpful README's.

Test Plan:
Use 'arcyd init' where appropriate in ./testbed/arcyd/exercise_arcyd.sh
$ ./precommit.sh
WIP: arcyd add-phabricator
WIP: arcyd add-repo
WIP: arcyd start
WIP: Arycd stop
testbed/arcyd/test_shell.sh: use new cli

Convert test_shell.sh to use the new Arcyd CLI

Test Plan:
$ ./testbed/arcyd/test_shell.sh
WIP: Arcyd: use pidfile to improve start/stop