dev: UDPConn implementation by @nickaxgit

[soypat]

• Added UDPConn, commented some areas for easier understanding, suggested a name change or two

• some tidying of udpConn.go

• Changes as requested (whitespace, UDP close method, extract anonymous function, errUnsupported returns, function comments etc)

• fixes two isuues spotted by b0ch3nski

• more linebreak issues fixed - appologies

• Tiny fix to itcphandler comment (although it has that c# smell)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 56.65529% with 127 lines in your changes missing coverage. Please review.

Project coverage is 46.15%. Comparing base (1201bab) to head (7d47ab1).

Files	Patch %	Lines
stacks/udpconn.go	42.71%	98 Missing and 16 partials :warning:
stacks/portstack.go	86.48%	4 Missing and 1 partial :warning:
stacks/port_udp.go	89.65%	2 Missing and 1 partial :warning:
stacks/ntp_client.go	0.00%	2 Missing :warning:
stacks/tcpconn.go	75.00%	2 Missing :warning:
stacks/dns_client.go	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #31 +/- ## ========================================== + Coverage 45.99% 46.15% +0.15% ========================================== Files 29 30 +1 Lines 4937 5163 +226 ========================================== + Hits 2271 2383 +112 - Misses 2406 2504 +98 - Partials 260 276 +16 ```

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[soypat]

@nickaxgit Fixed a bug in the checksum calculation- could you test this?

[nickaxgit]

Will do ... sorry life has been getting in the way a bit the last few days