remove dTHX usage
don't set global SP to itself after vcallsubn

the call_pv will already have moved/set global SP ahead of the PUSHMARK
position, so don't take the number of items put on the stack, then add
items to the mark then assign to global SP
Merge pull request #5 from bulk88/master

Proper dTHX -> pTHX_ macro conversion.  First round of some SP cleanup.