ftp: Rename ac_ accessors to ftp_

Using the same ac_ prefix as used for admin commands
is confusing and prevents the same class from providing
both ftp commands and admin commands.

This patch renames the ac_ methods to ftp_.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6295/