rsync: Add rrsync script

rsync: Add rrsync script

Rrsync is a perl script that is supplied as an extra with the rsync program.
It must be used in conjunction with openssh-server or openssh-server-pam
as it requires ~/.ssh/authorized_keys which is not supported by dropbear.

Rrsync allows selective access to subdirectories in either read-only, write-only or read-write,
depending on settings in authorized_keys. This allows for safe, restrictive access.
It's particularly useful for automated backup purposes.

An example usage would be this entry:

command="/usr/bin/rrsync -ro /home" <public key here>

This would allow a system connecting with this public key to be able to rsync FROM the
/home directory tree only. It could not write to this directory, nor read from any other directory.

Signed-off-by: Matt Reeve <redacted>