Add support for AVBL to built-in FTP Server

[azuravian]

I am putting together an app that supports connecting via FTP to Vita, and I was hoping to have support for showing the remaining free space on the SD Card or Internal Memory.

Some servers support this with their own implementation, such as WS_FTP supporting the XQUOTA command. There is a draft for AVBL command here: https://datatracker.ietf.org/doc/html/draft-peterson-streamlined-ftp-command-extensions-10#section-4

Any command would work, as long as the FTP server would return available bytes of free space somewhere in the response.