hv.c: Clarify uvar comment

It was not clear that it referred to uvar magic.
Stop delete local $ENV{foo} from leaking

It was only leaking when the env var did not already exist.

The code in question in pp.c:S_do_delete_local was calling hv_delete
to delete the element, which autovivifies, deletes, and returns a mag-
ical mortal for magical hashes.  It was assuming that if a value was
returned the element must have existed, so it was calling SvREFCNT_inc
on the returned mortal to de-mortalize it (since it has to be
restored).  Whether the element had existed previously was already
recorded in a bool named ‘preeminent’ (strange name).  This variable
should be checked before the SvREFCNT_inc.

I found the same bug in the array code path, potentially affecting
@- and @+, so I fixed it.  But I didn’t write a test, as that would
involve a custom regexp engine.