sftpserver: Support some openssh extensions

Add support for "hardlink@openssh.com" and
"posix-rename@openssh.com" extensions.

Signed-off-by: Chris Townsend <christopher.townsend@canonical.com>
Signed-off-by: Alberto Aguirre <albaguirre@gmail.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>