Fix regression that blocked copies of multiple files

The fix for the scp security vulnerability rejected any command
line with multiple non-option arguments in an attempt to block all
remote copies, but this introduced a regression that prevented
copying multiple files from a remote host.  Based on further
analysis of scp's command-line parsing, relax this requirement and
instead require that either -f or -t was given as an argument.
This should deactivate scp support for remote files.