Atomic print and re-print desc on status, flush before reboot

Refactor print() to emit description + final status in a single call to
cprintf(), preventing kernel messages from splitting the two parts.

For two-phase print(-1,...) + print_result() sequences used by, e.g.,
run_interactive, save the last description and re-print it before the
[ OK ] / [FAIL] output so the status is never left stranded on a blank
line when command output or kernel messages have scrolled away the
original description.

Finally, add print_exit() which drains the console output buffer with
tcdrain(2) and resets ANSI SGR attributes + cursor visibility before the
kernel takes back the console on reboot/halt, preventing escape code
leakage into bootloader or early-kernel output.

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

src/sig.c

@@ -424,14 +424,17 @@ void do_shutdown(shutop_t op)
 		}
 
 		print(0, "Rebooting ...");
+		print_exit();
 		reboot(RB_AUTOBOOT);
 	} else if (op == SHUT_OFF) {
 		print(0, "Powering down ...");
+		print_exit();
 		reboot(RB_POWER_OFF);
 	}
 
 	/* Also fallback if any of the other two fails */
 	print(0, "Halting ...");
+	print_exit();
 	reboot(RB_HALT_SYSTEM);
 }