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.
adjust $DB::deep from 100 to 1000

$DB::deep defaults to 100; this means that when the debugger gets 100
levels deep in subroutine calls, it automatically breaks execution.
While this might have been appropriate in 1989, in modern Perl programs,
the debugger might break before the program is even compiled.

This patch adjusts the default value up to 1000.

I was not able to find the default value documented anywhere.
version bumps and perldelta for debugger depth