lookip: Use line buffering for stdout

Otherwise, the output is buffered when e.g. piping the output to another
command (or file).  And it avoids having to call fflush() in the
interactive mode.

Fixes #3404.

src/libcharon/plugins/lookip/lookip.c

@@ -203,7 +203,6 @@ static int interactive(int fd)
 		int res;
 
 		printf("> ");
-		fflush(stdout);
 
 		if (fgets(line, sizeof(line), stdin))
 		{
@@ -266,6 +265,8 @@ int main(int argc, char *argv[])
 		return 1;
 	}
 
+	setvbuf(stdout, NULL, _IOLBF, 0);
+
 	if (argc == 1)
 	{
 		res = interactive(fd);