sftp: Add support for recent OpenSSH extensions

Looking into recent OpenSSH versions, there are several new extensions that are not handled in libssh (except for the ones discussed in #74 (closed) and #73 (closed)):

• copy-data (implemented as part of !383)
• home-directory (implemented as part of !473 (closed))
• users-groups-by-id@openssh.com
• expand-path@openssh.com (implemented as part of !419 (closed))
• limits@openssh.com (implemented as part of !413 (closed))
• lsetstat@openssh.com (implemeted as part of !455 (closed))

These should be described in https://github.com/openssh/openssh-portable/blob/master/PROTOCOL#L379

It would be good to review them and implement what would make sense to keep up with openssh changes.