Use int64_t instead of off_t

Using off_t, at least, in DiskAdaptor layer is problematic because
torrent can contain under 2GiB files but total sum of those files may
exceed 2GiB limit, which makes off_t overflow in 32 bit system without
large file support. So we use int64_t in API. We'll check the file
length before download so that it does not exceed max off_t.

test/SingleFileAllocationIteratorTest.cc

@@ -35,8 +35,8 @@ void SingleFileAllocationIteratorTest::testAllocate()
   CPPUNIT_ASSERT_EQUAL((int64_t)10, x.size());
 
   DefaultDiskWriter writer(fn);
-  off_t offset = 10;
-  off_t totalLength = 16*1024*2+8*1024;
+  int64_t offset = 10;
+  int64_t totalLength = 16*1024*2+8*1024;
 
   // we have to open file first.
   writer.openExistingFile();