Hooks take new "modules" argument, and hook names can contain periods.

* debian/mandos-client.README.Debian: Adjust wording.
* debian/rules (binary-common): Exclude nework-hooks.d directory from
                                dh_fixperms.
* initramfs-tools-hook: Also create network hook directory.  Allow
                        periods in hook names.  Only run executable
                        files in hook directory.  Copy needed modules.
* network-hooks.d/bridge: Fix "/usr/bin/brctl" to "/usr/sbin/brctl".
                          Also take "modules" argument.
* plugins.d/mandos-client.xml (NETWORK HOOKS/REQUIREMENTS): Document
                                                            the
                                                            allowing
                                                            of periods
                                                            in hook
                                                            names.
  (NETWORK HOOKS/REQUIREMENTS/files): Adjust wording.
  (NETWORK HOOKS/REQUIREMENTS/modules): New.
  (NETWORK HOOKS/REQUIREMENTS/MANDOSNETHOOKDIR): Clarify.
  (NETWORK HOOKS/REQUIREMENTS/MODE): Add "modules".