ftpd: fix the bug where >2GB file ops report errors;

ftpd: fix the bug where >2GB file ops report errors;
 make a few simplifications; add TODOs.

function                                             old     new   delta
port_or_pasv_was_seen                                  -      37     +37
get_remote_transfer_fd                               104     109      +5
handle_upload_common                                 265     260      -5
handle_dir_common                                    228     223      -5
popen_ls                                             211     203      -8
ftpd_main                                           1825    1815     -10
data_transfer_checks_ok                               37       -     -37
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(add/remove: 1/1 grow/shrink: 1/4 up/down: 42/-65)            Total: -23 bytes