support 'whence' arg to seek()

Lib/mailbox.py

@@ -12,8 +12,13 @@ class _Mailbox:
 		self.fp = fp
 		self.seekp = 0
 
-	def seek(self, pos):
-		self.seekp = pos
+	def seek(self, pos, whence=0):
+		if whence==1:		# Relative to current position
+			self.pos = self.pos + pos
+		if whence==2:		# Relative to file's end
+			self.pos = self.stop + pos
+		else:			# Default - absolute position
+			self.pos = self.start + pos
 
 	def next(self):
 		while 1: