Add the lz4-v2 compression option, which is currently (version 2.4.8)
not documented in the man page but apparently supported since years
[1].

The difference with lz4 is "packet data alignment, which is better
with the -v2 compression algorithms - which reflects into 'less CPU
usage, less power drain on mobile devices, possibly higher
throughput'".

[1] https://community.openvpn.net/openvpn/ticket/820