ftp: abort function optimalization

It seems some ftp servers doesn't respect ABOR command,
but closing both connection is slow.

This commit keeps control connection open when possible.

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>
ftp: fix seeking beyond file size

adjust to ff* tools seek nature

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>
ftp: probe seek capability

Make FTP streamed when server doesn't accept REST command

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>
ftp: add invalid code for RETR operation

554 is possible invalid code:
- Restart not valid
- Command terminated due to server shutdown in progress
- etc...

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>