Refactor Get-BTDanglingSPN to fix bugs and make it zoom

[SamErde]

All of the updates. ☕

• Refactored to eliminate redundant DNS checks and rely on a cached copy of DNS records.
• Added a separate check to resolve FQDNs in external/excluded domains.
• Using more gatekeeping if blocks to reduce length of checks for each principal and SPN.
• Fixed bug that included SPNs with port numbers (eg: SQL) as hostnames.
• Added comment-based help because it's awesome.
• Clarified property names in the DanglingSPN output.