Add support for yorn(), safe yes-or-no prompt

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

tests/yorn.c

@@ -0,0 +1,32 @@
+#include <stdio.h>
+#include <unistd.h>
+#include <sys/wait.h>
+
+#include "check.h"
+
+int main(int argc, char *argv[])
+{
+	int chan[2];
+	int status;
+
+	pipe(chan);
+
+	if (fork() > 0) {
+		dup2(chan[0], STDIN_FILENO);
+		if (yorn("Do you really wanna do this (y/N)? ")) {
+			printf("Affirmative!\n");
+			_exit(0);
+		}
+
+		printf("OK, aborting!\n");
+		_exit(1);
+	}
+
+	sleep(1);
+	if (write(chan[1], "Y", 1) != 1)
+		return 1;
+
+	wait(&status);
+
+	return WEXITSTATUS(status);
+}