Add more optional guards against race conditions to mirror

[MrVoltz]

Hi, I am using lftp to mirror a data directory of a running application from a remote SFTP server, which means that files can be modified during the mirror operation.

Currently, mirror only detects, when size of the remote file decreases, but doesn't detect when the size increases or the modification time changes. Would it be possible to add those two additional checks under some option?