#229 Blake2 in place of MD4 with updated prerequisite on librsync

[Denis Corbin]

check if possible to replace MD4 by BLAKE2 in librsync (generic_rsync.cpp line 92)

[Denis Corbin]

Quoted from Anthony's email:

Actually i was referring to the rsync signatures : librsync supports md4
or blake2 for the strong signatures and adler32 or rabinkarp for the
rolling checksum.
https://librsync.github.io/librsync_8h.html#a226f57577fcad2e4c97f3e4d612650d3
https://librsync.github.io/librsync_8h.html#ab6abafd1db0a3947bb3c076507f4f7c2

[Denis Corbin]

consider next archive format to add two parameters in archive header, one for signature hash algo and another form rolling checksum algo. And default to the MD4 for older format.
See rs_sig_magic() doc and https://librsync.github.io/librsync_8h.html#ab6abafd1db0a3947bb3c076507f4f7c2