Fix @ISA recursion during global destruction

Fix @ISA recursion during global destruction

With -DDEBUGGING and make test (not make test_harness):

Failed 3 tests out of 1889, 99.84% okay.
	mro/basic.t
	mro/recursion_c3.t
	mro/recursion_dfs.t

See <http://www.nntp.perl.org/group/perl.perl5.porters/2010/10/msg165311.html>.

This was broken by change 35759254, which made S_hv_delete_common call
mro_package_moved when the G_DISCARD flag was passed. It used to be
skipped, which was a mistake.

This commit simply skips mro_package_moved when $::{"main::"} is
deleted. It was buggy anyway (main::E being passed to recursive calls,
instead of E), and should only happen at global destruction. (If
$::{"main::"} is deleted at run-time, it stop Perl from being func-
tional, so no one should be doing that.)