A signed-unsigned conversion was making the
PASV command sometimes report signed numbers.
This was detected by the Windows firewall as
a problematice connection and it therefore
terminated the connection.

An FTP client implementation has been
observed to close the data connection
as soon as it receives the 226 status
code - even though it hasn't received
all data, yet. To improve interoper
with such buggy clients, sending of
the 226 status code can now be delayed
a bit. The optional delay is controlled
by means of preprocessor define that
can be set from the cmake command
line through a cmake cache variable.

The data socket is now also closed
explicitly after file fetch in the
case where sending of code 226 is
not delayed.