Only print prompt in interactive usage

Only print prompt in interactive usage

Scripting tftp(1) makes it non-interactive, yet the prompt is still
printed and may mess up the shell's PS1:
	$ echo put nonexistent | tftp localhost
	tftp> tftp: open: nonexistent: No such file or directory
	tftp> $

The fix seems easy and works as expected for multiple commands as well:
	$ echo 'verbose\nput nonexistent' | ./obj/tftp localhost
	Verbose mode on.
	tftp: open: nonexistent: No such file or directory
	$

OK millert