hscontrol: route hostname handling through dnsname and NodeStore

Ingest (registration and MapRequest updates) now calls
dnsname.SanitizeHostname directly and lets NodeStore auto-bump on
collision. Admin rename uses dnsname.ValidLabel + SetGivenName so
conflicts are surfaced to the caller instead of silently mutated.

Three duplicate invalidDNSRegex definitions, the old NormaliseHostname
and ValidateHostname helpers, EnsureHostname, InvalidString,
ApplyHostnameFromHostInfo, GivenNameHasBeenChanged, generateGivenName
and EnsureUniqueGivenName are removed along with their tests.
ValidateHostname's username half is retained as ValidateUsername for
users.go.

The SaaS-matching collision rule replaces the random "invalid-xxxxxx"
fallback and the 8-character hash suffix; the empty-input fallback is
the literal "node". TestUpdateHostnameFromClient now exercises the
rewrite end-to-end with awkward macOS/Windows names.

Fixes #3188
Fixes #2926
Fixes #2343
Fixes #2762
Fixes #2449
Updates #2177
Updates #2121
Updates #363