feat: add multicast UDP listener support

[leki75]

1. Are you opening this pull request for bug-fixes, optimizations or new feature?

New feature

2. Please describe how these code changes achieve your intention.

This PR is intended to support UDP multicast listeners. Whenever a user wants to bind to a UDP multicast address one needs to provide additional setsockopt calls to make it work.

3. Please link to the relevant issues (if any).

4. Which documentation changes (if any) need to be made/updated because of this PR?

Additional configuration option: WithMuticastInterfaceIndex.
Examples:

gnet.Run(echo, "udp://224.0.0.100:8765"))

or

iface, _ := net.InterfaceByName("eth0")
gnet.Run(echo, "udp://224.0.0.100:8765"), gnet.WithMulticastInterfaceIndex(iface.Index))

4. Checklist

• I have squashed all insignificant commits.
• I have commented my code for explaining package types, values, functions, and non-obvious lines.
• I have written unit tests and verified that all tests passes (if needed).
• I have documented feature info on the README (only when this PR is adding a new feature).
• (optional) I am willing to help maintain this change if there are issues with it later.

[panjf2000]

Thanks for this feature, please add some tests for it.

[codecov]

Codecov Report

Base: 71.26% // Head: 71.42% // Increases project coverage by +0.16% 🎉

Coverage data is based on head (aca68c9) compared to base (20ff40b).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head aca68c9 differs from pull request most recent head 91f25e7. Consider uploading reports for the commit 91f25e7 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #412      +/-   ##
==========================================
+ Coverage   71.26%   71.42%   +0.16%     
==========================================
  Files          13       13              
  Lines        1392     1428      +36     
==========================================
+ Hits          992     1020      +28     
- Misses        328      335       +7     
- Partials       72       73       +1     

Flag	Coverage Δ
unittests	71.42% <100.00%> (+0.16%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
listener.go	100.00% <100.00%> (ø)
options.go	94.87% <100.00%> (+0.27%)	⬆️
gnet.go	71.22% <0.00%> (-3.34%)	⬇️
connection.go	66.14% <0.00%> (+0.94%)	⬆️

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[panjf2000]

Based on the failures on Linux runners, it seemed that there is no expected loopback interface with multicast support on Github Linux runners, whereas there is on macOS runners, so I assume we need to skip these tests when the OS is Linux?

[leki75]

Sorry for making so many changes. It is hard to write a test that runs both on Linux and Mac. The interface names and capabilities differ. Eg. the loopback interface name is lo0 and the multicast flag is set on Mac, while it is lo on Linux and the flag is not set. Writing a universal networking library is really hard! My appreciation.

[panjf2000]

Sorry for making so many changes. It is hard to write a test that runs both on Linux and Mac. The interface names and capabilities differ. Eg. the loopback interface name is lo0 and the multicast flag is set on Mac, while it is lo on Linux and the flag is not set. Writing a universal networking library is really hard! My appreciation.

Don't worry about it, all these commits will be squashed into one commit finally.

[panjf2000]

Thanks~