increase the per-peer prefetch count from 12 to 18. Add a symbolic constant for it.

git-svn-id: svn://svn.transmissionbt.com/Transmission/trunk@13782 f4695dd4-2c0a-0410-b89c-da849a56a58e
minor code simplification in torrent.c's deleteLocalData() function.

git-svn-id: svn://svn.transmissionbt.com/Transmission/trunk@13783 f4695dd4-2c0a-0410-b89c-da849a56a58e
(trunk) bump to 2.76+

git-svn-id: svn://svn.transmissionbt.com/Transmission/trunk@13786 f4695dd4-2c0a-0410-b89c-da849a56a58e