Print stacktrace in SIGINT handler if RESTIC_DEBUG_STACKTRACE_SIGINT set

The builtin mechanism to capture a stacktrace in Go is to send a SIGQUIT
to the running process. However, this mechanism is not avaiable on
Windows. Thus, tweak the SIGINT handler to dump a stacktrace if the
environment variable `RESTIC_DEBUG_STACKTRACE_SIGINT` is set.

internal/debug/stacktrace.go

@@ -0,0 +1,15 @@
+package debug
+
+import "runtime"
+
+func DumpStacktrace() string {
+	buf := make([]byte, 128*1024)
+
+	for {
+		l := runtime.Stack(buf, true)
+		if l < len(buf) {
+			return string(buf[:l])
+		}
+		buf = make([]byte, len(buf)*2)
+	}
+}