Behavior:
dns.setServers throws a null pointer dereference on some inputs.
Expected behavior was the more pleasant
  TypeError [ERR_INVALID_IP_ADDRESS] ...

Root cause(s?):
- Dereferencing the result of a regex match without confirming
  that there was a match.
- assuming the capture of an optional group (?)

Solution:
Confirm the match, and handle a missing port cleanly.

Tests: I added tests for various unusual inputs.

Fixes: nodejs#20441

Problem:
The IPv6 regex was quadratic.
On long malicious input the event loop could block.

The security team did not deem it a security risk,
but said a PR was welcome.

Solution:
Revise the regex to a linear-complexity version.

Tests:
I added REDOS tests to the "oddities" section.

Fixes: nodejs#20443