Blake2 in place of MD4 with updated prerequisite on librsync

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

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

implemented, see as overloaded method of of archive_option_{create|isolate|merge}::set_delta_signature() with the new rsync_sig_magic enum. At CLI level the "--delta sig:...." receives an additional argument to define either blake2 or rk-blake2 (the now default) hash algorithm to build delta signatures.

The archive listing has been enhanced to display the hash algorithm used for the signature where [D] stands for md4 (backward compatible with the older versions, [B] for blake2 and [R] for rk-blake2.