Add support for FTP open/read commands
Support for Open/Read/Terminate FTP commands

Also modified protocol to better support eof’s on list and read
commands.
Switch to kErrEOF style read loop handling
Fix warning from Linux/Windows build
Workaround odd Windows std::string problem
Remove missed qDebug
Merge pull request #733 from DonLakeFlyer/mavlink-ftp

Mavlink FTP: List and Download commands working end to end