Fix inconsistent $proxy_host handling in apt and apt::ppa.

- The default for $proxy_host is undef
- apt considers $proxy_set to be absent if $proxy_host is undef
- apt::ppa considers proxy_env to be empty if $proxy_host is false or ''

This results in apt::ppa to consider $proxy_host to be set when the default undef is used
breaking ppa resources because $proxy_env becomes:
  [http_proxy=http://:8080, https_proxy=http://:8080]

Fix this by making both apt and apt::ppa consider $proxy_host to be unset when it is
any of false, '', or undef.
Fix mistake in my 0c2329b implementation.
Fix readme typo.
Fix broken acceptance tests.

New fact was added that matched a regex breaking the always_apt_update
tests.  Updated the tests to check for the apt_update exec, not just the
string apt_update.
Debian seems to have hanging apt-get and/or dpkg processes, so kill
those.