netcat: ensure that there aren't spurious cmdline arguments
netcat: ensure that there aren't spurious cmdline arguments

Ensure that flag.Args() == 1, exit with usage output and error otherwise.

Signed-off-by: Matteo Croce <02599495dea0e5fe5ba2d24686ec50bb27d90a2c@redhat.com>

rminnich/u-root,GanShun/u-root,rminnich/u-root,hugelgupf/u-root,u-root/u-root,u-root/u-root,hugelgupf/u-root,rminnich/u-root,u-root/u-root,GanShun/u-root,hugelgupf/u-root,rminnich/u-root,GanShun/u-root,hugelgupf/u-root