Add a new default_streamid configuration option for the SRT server to
allow customization of the default streamid used when SRT clients
connect without setting the SRTO_STREAMID socket option.
When SRT clients (like VLC) connect without properly setting the
SRTO_STREAMID socket option, SRS previously used a hardcoded default
value of "#!::r=live/livestream,m=publish". This caused conflicts when:
1. A publisher connects without setting streamid (uses default publish
mode)
2. A player tries to connect without setting streamid (also uses default
publish mode)
3. The player gets a "Stream already exists or busy" error because both
are trying to publish
Start SRS with default stream id as viewer:
```bash
./objs/srs -c conf/srt.vlc.conf
```
Publish SRT stream by FFmpeg:
```bash
ffmpeg -re -i ./doc/source.flv -c copy -pes_payload_size 0 -f mpegts \
'srt://127.0.0.1:10080?streamid=#!::r=live/livestream,m=publish'
```
Play SRT stream by ffplay:
```bash
ffplay 'srt://127.0.0.1:10080?streamid=#!::r=live/livestream,m=request'
```
Play SRT stream by VLC:
```
✅ Correct - Use simple URL without streamid parameter:
srt://127.0.0.1:10080
❌ Wrong - VLC doesn't support streamid parameter:
srt://127.0.0.1:10080?streamid=#!::r=live/livestream,m=request
```
---
Co-authored-by: OSSRS-AI <winlinam@gmail.com>
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