Add enum for parse_name() return value

Summary:
Currently, parse_name() returns an int that doesn't describe what has
actually happened (at least without reading the comment that was in
dns.h). This changes the return value to something more descriptive.

Depends on D5470

Test Plan:

ninja check-bitcoin-seeder

Run sanitizer builds

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox, deadalnix

Subscribers: deadalnix, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5471