&CORE::unpack()

&CORE::unpack()

This commit allows &CORE::unpack to be called through references and
via ampersand syntax.

It moves the $_-handling code in pp_coreargs inside the parameter
loop, so it can apply to the second parameter, not just the first.
Consequently, a mkdir test has been added that ensures implicit $_
is not used for mkdir’s second argument; i.e., that the $_-handling
code’s if() condition is correct.