Restore special blocks to working order

Restore special blocks to working order

I accidentally broke these in commit 85ffec3682, yet everything passed
for me under threads+mad.

PL_compcv is usually restored to its previous value at the end of
newATTRSUB when LEAVE_SCOPE is called.  But BEGIN blocks are called
before that.  I needed PL_compcv to be restored to its previ-
ous value before it was called, so I added LEAVE_SCOPE before
process_special_blocks.

But that caused the name to be freed before S_process_special_blocks
got a chance to look at it.

So I have now added a new parameter to S_process_special_blocks to
allow *it* to call LEAVE_SCOPE after it determines that it is a BEGIN
block, but before it calls it.