Integrate perlio:

[ 13808]
Add pTHX to all vtable functions for future-proofing and
consistency of PerlIO layers.

[ 13797]
Remove the sometimes-lvalue cast

[ 13796]
alarm.t with -Uuseperlio fix
- SA_RESTART should only be function of PERL_OLD_SIGNALS
not USE_PERLIO

[ 13795]
Fix READONLY hashes:
1. Discovered need for disabled scope.c part of Jeff's patch.
2. Corrected hv_delete* - entry was being de-linked from
chain (patch program not smart enough?)

p4raw-link: @13808 on //depot/perlio: f62ce20a4126b1e303e2d4d0a5c1e049ef2cb0c2
p4raw-link: @13797 on //depot/perlio: 205c8ad3acbda0df8cac03a0c7e619f1855229a8
p4raw-link: @13796 on //depot/perlio: 0dd95eb270db2cdad9a8acda96ac3a3aab8775c2
p4raw-link: @13795 on //depot/perlio: a26e96dff1c9d91b0bf389e6149016d8a3f26e49

p4raw-id: //depot/perl@13809