Improve support for $ftp->supported(CMD)

Enterprise FTP server (version 6600) returns a message beginning with "The
following ftp server commands are recognized:" in response to a HELP
command, which caused $ftp->supported('ALLO') to return 1 when in fact it
should (surprisingly!) return 0.
Don't use the ALLO command on FTP servers that don't support it

Despite always having been present in the FTP specification, some servers
(e.g. Enterprise FTP server (version 6600)) do not support the ALLO
command. Do not use the command on such servers otherwise its failure will
cause the PUT command which normally follows it to fail (since the success
of the ALLO command is now checked, as of commit 0d616e68cc).

Fixes CPAN RT#95717, also reported by an Enterprise FTP server user.