fix(cli): parsing of default IPv6 route (#447)

[0x455241]

PR type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other... Please describe:

PR checklist

Please check if your PR fulfills the following requirements:

• I have read the CONTRIBUTING.md doc
• The Git workflow follows our guidelines: CONTRIBUTING.md#git
• I have added necessary documentation (if appropriate)

What is the current behavior?

$ ivpn exclude COMMAND
Error: Failed to enable Split Tunnel: (exit status 2) ip6tables v1.8.13 (nf_tables): interface name `xxxx::xxxx:xxxx:xxxx:xxxx' must be shorter than 16 characters
Try `ip6tables -h' or 'ip6tables --help' for more information.

Issue number: #447

What is the new behavior?

$ ivpn exclude COMMAND
Running command in Split Tunneling environment (pid:XXXXXX): COMMAND

Does this PR introduce a breaking change?

• Yes
• No

Other information