3.1.x branch compatibility issue with html5 v5.x

[totaam]

When running against a v3.1.x server, there is only rencode and bencode packet encoders available and so the server chooses bencode.

info-response packets trigger a network parsing error because the network parsing code in the html5 client has been re-worked to prefer rencodeplus (#3229) and not use rencode (too many unfixable problems).

This is not easily fixable - especially in an EOL branch. Potential solutions:

• don't send info-response packets with bencode (+ html5 client?)
• add rencodeplus to the v3.1.x branch

[totaam]

Sample packet causing parsing error in html5 client : html5-bdecode-packet.txt

The commit above adds rencodeplus - this may still be reverted if this causes problems.
For a start, this also required a fix to the v5.x branch of the html5 client: Xpra-org/xpra-html5@67259c4

[totaam]

Make it easy to drop rencodeplus in case of problems:

• run with --packet-encoders=bencode,rencode
• nuke the rencodeplus directory in the python lib folder
  Both work.