dns queries with underscores are refused

[scareything]

Validation in the DNS server prevents lookups for hostnames with '_' (and other non-alphanumeric chars) from being handled.

1. Underscores are commonly used in SRV queries

2. Section 11 of RFC 2181 clarifies that DNS servers should not refuse queries (or loading zones) due to characters in the labels.

   In particular, DNS servers must not refuse to serve a zone because it contains labels that might not be acceptable to some DNS client programs. A DNS server may be configurable to issue warnings when loading, or even to refuse to load, a primary zone containing labels that might be considered questionable, however this should not happen by default.

Based on this, we should remove the character validation in check_names.