Handle KeyboardInterrupt quietly, fix signal return codes

This is a continuation of #8.

The previous patch handled BrokenPipeError, this one makes it so that
KeyboardInterrupt is also handled. This is relevant when pressing ^C on
very large outputs (e.g. big files with --rows 999999). If such use of
^C occurs while in a pager, broken pipe errors were also printed by the
Python exit error handler.

This patch makes it so that both BrokenPipeError and KeyboardInterrupt
exit quietly.

As a drive-by fix, I realized that the exit codes were not following
the convention typically used e.g. by shells. The new code follows such
convention.

Author: ntrrgc

mp4parser.py

@@ -3,7 +3,7 @@
 Core parsing logic
 '''
 
-from errno import EPIPE
+import os
 import sys
 import mmap
 import itertools
@@ -252,7 +252,7 @@ def subparser(self, n: int):
 	def handle_errors(self):
 		try:
 			with self: yield self
-		except BrokenPipeError:
+		except (BrokenPipeError, KeyboardInterrupt) as err:
 			# All of this is necessary so that no exceptions are printed when
 			# the user pipes `mp4parser --rows 1000 large_file.mp4 | less` and
 			# quits with 'q' before getting close to the end of the file.
@@ -271,7 +271,15 @@ def handle_errors(self):
 				sys.stdout.close()
 			except BrokenPipeError:
 				pass
-			raise SystemExit(EPIPE)
+			if os.name == "posix":
+				from signal import SIGINT, SIGPIPE
+			else:
+				# If we're in Windows or any other platform without signal ids,
+				# use the Linux x86 signal ids, just to give something helpful.
+				SIGINT, SIGPIPE = 2, 13
+			# Standard exit codes are 128 + signal id.
+			exit_code = 128 + (SIGINT if isinstance(err, KeyboardInterrupt) else SIGPIPE)
+			raise SystemExit(exit_code)
 		except Exception as e:
 			print_error(e, self.prefix)
 			if max_dump and self.buffer: