Support active charset setting through "OPTS" FTP command.

Currently, Solid Explorer support select various charset in storage setting for ftp server. But, it's just passive charset setting (not requesting charset to server, only setting about which charset using for interpreting received file list from server)

This method is well this method well functions in many ftp servers without ftp server on IIS (on windows system). Most IIS FTP server in non-English area response file list with local charset like EUC-KR, Shift-JIS. And this cause problem that files named with CJK characters don't displayed correctly and most file operation cannot be conducted correctly.

Solution for this problem is send "OPTS UTF8 ON" Command to server when handshake process. Then, both server and client use utf-8 charset and CJK characters will display correctly. As result, add additional encoding option on storage setting and when it is selected send OPTS command to support opts command!

Thanks for reading.