5.003_18: perl_{con,des}truct fixes

5.003_18: perl_{con,des}truct fixes

Here's my current patch to clean/reset global variables.  I trust more
work needs to be done in this area, there are comments that might help
if something pops up down the line.

I did not test MULTIPLICITY extensively, but a simple test case seems to
work, I've added that to perlembed.pod.

A *huge* _thank you_ to Chip for your help in this area, 5.004 will make
many once frusterated embedders very happy!

p5p-msgid: <199701032042.PAA06766@postman.osf.org>