stop assuming the interrupt key is [ctrl-C]

The recent discussion around allowing the user to change various termios
(i.e., stty) settings reminded me that there are places in our code
where we assume the interrupt key is [ctrl-C]. That's a bad assumption.
Instead use the actual value reported to us by the kernel.

This also makes the fkr program friendlier by always reporting when a
signal was received, not just when run with -d2, and prompting the user
to press the INTR or EOF key a second time to exit.

src/input.cpp

@@ -4,6 +4,7 @@
 #include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <termios.h>
 #include <unistd.h>
 #include <wchar.h>
 #include <wctype.h>
@@ -291,8 +292,7 @@ static int interrupt_handler() {
     if (job_reap(1)) reader_repaint_needed();
     // Tell the reader an event occured.
     if (reader_reading_interrupted()) {
-        // Return 3, i.e. the character read by a Control-C.
-        return 3;
+        return shell_modes.c_cc[VINTR];
     }
 
     return R_NULL;