Rewrite thread return code to distinguish between ordinary return

Rewrite thread return code to distinguish between ordinary return
and die() and make join propagate the die. Add tiny method eval
which just does "return eval { shift->join; }". Add Thread::Specific
class for access to thread specific user data along with specific.t.
Rename Class to classname throughout Thread.xs for consistency.
Fix pp_specific to pp_threadsv in global.sym. Add support to
pp_entersub in pp_hot.c to lock stash for static locked methods.

p4raw-id: //depot/perl@248