Double COVID vaccinated

As per 2.0, with several bugfixes ( with thanks to @enoch85 ):

• Fixed references to chain being removed from INPUT and FORWARD when updating.
• Now only attempt to remove references to chain from INPUT and FORWARD if they exist.
• Silenced some (expected) stderr messages from iptables
• Fixing incorrect reference, and correcting a typo + missing explanation in an error message.
• Make sure that the message on success only is written if it's actually successful.
• Fixed "Too many links" from iptables (not actually removing the chain) by flushing the chain before removal.

COVID vaccine ready

Fundamentally the same, but now the script has a better user interface.

New CLI arguments allow various behaviours not possible before:

Basic usage: spamhaus-drop <-u>

Additional options and arguments:
  -u                   Download blocklists and update iptables
  -c=CHAIN_NAME        Override default iptables chain name
  -l=URL_LIST          Override default block list URLs [careful!]
  -f=CACHE_FILE_PATH   Override default cache file path
  -s                   Skip failed blocklist downloads, continuing instead of aborting
  -z                   Update the blocklist from the local cache, don't download new entries
  -d                   Delete the iptables chain (removing all blocklists)
  -o                   Only download the blocklists, don't update iptables
  -t                   Disable logging of blocklist hits in iptables
  -h                   Display this help message