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:

./objs/srs -c conf/srt.vlc.conf

Publish SRT stream by FFmpeg:

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:

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