Add 6rd tunnel functionality.

Introduces a hook that can connect to 6rd border relay servers.
Exit if the check functionality of a hook fails.

Introduce the exit with a critical error if the check function of a hook fails.
Merge branch 'master' into 6rd-new
6rd: Rename hook functions to match the latest function naming scheme.
6rd: Change cmdline option to public-ipv4-address.

I change the command line option for the public IPv4 address from "--public-address" to
"--public-ipv4-address" to clarify the required input data.
6rd: Add documentation.
Merge remote-tracking branch 'stevee/error-if-check-fails'