Interface and gateway name/address filters

The purpose of this is to decrease the need to manually disable the
cronjobs on some networks (like hotel WiFi or 3G/4G).

For now, this only covers the simplest use case, that of someone
monitoring one (or more) specific connections.
Makefile: no sudo & separate crontab and install

First, the need for root/sudo should be explicit.

Second, since the crontab edited is usually not the root one, it makes
sense to separate the two targets. Also, with the new filter options,
more finer control over the crontab entries will be more common.