move fac.py and associated tests to flashproxy-common
- use the facilitator's parse_addr_spec, and fix client uses to set defhost = ""
  - the old client side accepted an empty host when defhost = None.
  - the facilitator side requires setting defhost = "" to accept an empty host
  - behaviour is otherwise equivalent
use common certificate pinning code in facilitator-email-poller
in extra-install tasks, use ${} rather than $() syntax for easier copying into distro-specific scripts
more specific rm in remove-{secrets,symlinks} and add remove-daemon-data to purge logs and other data
- also don't symlink no-longer-existant appengine/README
document and add tests for the None/"" default behaviour of parse_addr_spec