The aim is to make php handlers not always required when there is a FTP
server that supports "SITE MD5" commands (like proftpd with mod_md5).
This commit makes FtpServerHandler able to handle such feature and avoid
using any other external hook. If server support is found, this verification
is enough. If support is broken, FtpServerHandler will try to fallback to
previously supported features, if enabled (use_handlers variable).
Moreover, FtpServerHandler now makes use of entropy.tools.is_valid_md5
instead of just checking if provided strings are long 32 chars.
a97e726622