Fix infinite loop when the viewer exits

* The viewer now sends `SIGHUP` to the main process when it exists
* The main process signal handler kills the viewer and terminates the
  notifier/main event loop when receiving `SIGINT`/`SIGTERM`/`SIGQUIT`
* The main process signal handler just terminates the notifier/main
  event loop when receiving `SIGHUP`
* Clean just after the notifier/main loop finishes

rubberin

@@ -172,18 +172,9 @@ viewer_pid =
     view(infile, mode)
     # If xdvi/evince exits, this program should exit too.
     puts "#{PROGRAM}: viewer exited, so will I!"
-    Process.kill("TERM", pid)
+    Process.kill("HUP", pid)
   end
 
-# Handle signals.
-["INT", "TERM", "QUIT"].each do |sig|
-  Signal.trap(sig) do
-    Process.kill(sig, viewer_pid)
-    clean(infile)
-    exit
-  end
-end
-
 ## Main event loop
 
 # Add input file with dependancies to the watch list and start event loop.
@@ -203,4 +194,18 @@ dirs.each do |dir|
   end
 end
 
+# Handle signals during the main event loop.
+["INT", "TERM", "QUIT"].each do |sig|
+  Signal.trap(sig) do
+    puts "#{PROGRAM}: caught signal #{sig}, stopping the viewer..."
+    Process.kill(sig, viewer_pid)
+    notifier.stop
+  end
+end
+Signal.trap("HUP") { notifier.stop }
+
+# Run the main event loop.
 notifier.run
+
+# Clean before finishing!
+clean(infile)