Integrate perlio:

[  8401]
Allow control of 2nd arg to segsetjmp() via -DSCOPE_SAVES_SIGNAL_MASK.
(So Alan and Ilya can try setting via hints.)
Verified Linux "works" with it true and fails to re-enable SIGINT if false.
Restoring sigmask in Perl_sighandler() if handler dies is not equivalent;
doing the save everywhere seems to restart read() correctly, but just
doing it once "round" the handler does not.

[  8399]
Safe fix for Simon's pclose() doing SvIVX of undef -> core bug.

[  8393]
Back out Simon's pid hackery for now.

[  8371]
Fix "scalars leaked" bugs caused by overload magic
(Highlighted by Ilya's DESTROY optimization.)

p4raw-link: @8401 on //depot/perlio: b03c0a3af9d8dc230dbf7f55955fdc7ce65f2403
p4raw-link: @8399 on //depot/perlio: 25d920232779109764865e51a54577792f1390e3
p4raw-link: @8393 on //depot/perlio: 245f541b6f35ec487dff355c77302d41b2c043a6
p4raw-link: @8371 on //depot/perlio: d460ef459c7692518f607c250b9843bea7e01dd6

p4raw-id: //depot/perl@8402