Honor PGHOST and PGPORT when omitted from database URLs

[tianrking]

Summary

• keep PostgreSQL host and port components out of dbmate's rewritten URL when PGHOST or PGPORT supplies the missing value
• preserve explicit URL/query values and the Redshift 5439 default without emitting an empty hostname that overrides PGHOST
• cover independent and combined environment values, URL precedence, and the final address selected by lib/pq

Closes #492.

Testing

• go test ./pkg/driver/postgres -run '^TestConnectionString' -count=1
• go test ./pkg/driver/postgres -count=1
• go vet ./pkg/driver/postgres
• git diff --check