Integrate from perlio:

[ 18560]
Fixes for open.pm which attempts to load layers:
 1. C equivalent of local $SIG{__WARN__} = sub {}
    while loading layers to supress warnings lib/open.t does
    not want.
 2. The loading scheme does not recurse now so look for
    new symptom of bad layer which is that a good module
    fails to load (as we cannot open any files).

NOTE: In my opinion open.pm should probably die on bad layer
spec rather than just (maybe) warning and then allowing opens
to fail.
p4raw-link: @18560 on //depot/perlio: c9bca74aca217023baf0f921dcffaaa072a83cf3

p4raw-id: //depot/perl@18561

diff --cc perlio.c
Simple merge