FTP: Add new open command for write.

All open commands now return file size.
FTP: Implement write command.
FTP: Add truncate command.

Unfortunately NuttX not provides truncate(),
had to be emulated by copying with O_TRUNC flag.
FTP: fix truncate errors. Also correct errno reporting.

Seems that warnx() everytime changes originak errno to EINVAL.
FTP: Rename command.

Payload: `<oldpath>\0<newpath>`
See: man 2 rename
FTP: Add file checksum calculation command.
FTP: Save errno in _copy_file().
FTP: Return bytes written in payload.
Merge pull request #1363 from vooon/ftp_write_support

FTP: write support