Integrate changes #8608,8609,8610,8611,8612,8613 from maintperl.

Integrate changes #8608,8609,8610,8611,8612,8613 from maintperl.

perl's internal variables are not for public consumption,
move their docs from perlapi.pod to perlintern.pod

support for -Dusethreads build under HP-UX 10.20 and DCE threads
library (11.0 and later have pthreads but 10.x don't)

avoid uninitialized value warnings

canonicalize paths when doing chdir() on windows (or Cwd::getcwd()
gets weird results)

perldoc nit on windows

make it possible to run the tests outside the source tree
(there's still a dependency on ../lib being the library
location)

p4raw-link: @8612 on //depot/maint-5.6/perl: 21b80573951053245624647c6ea0077acae22b99
p4raw-link: @8611 on //depot/maint-5.6/perl: 706e8e4d2d0a6985197cb1cb76dc3d2575cd159e
p4raw-link: @8610 on //depot/maint-5.6/perl: fc1aae957f18d75529c72450055ab0d424b7d318
p4raw-link: @8609 on //depot/maint-5.6/perl: 13b98125bf15a4dfacaa2c6f56d3742c97180500
p4raw-link: @8608 on //depot/maint-5.6/perl: f893c726cdaa1ccdec167db9b2ff5c2fdc19e29b

p4raw-id: //depot/perl@8614
p4raw-integrated: from //depot/maint-5.6/perl@8596 'copy in'
	t/lib/dprof/V.pm t/op/flip.t utils/perldoc.PL (@8159..)
	win32/bin/search.pl (@8166..) t/io/tell.t (@8175..)
	hints/hpux.sh (@8601..) thread.h (@8606..) 'ignore'
	pod/perlintern.pod (@7766..) pod/perlapi.pod (@8605..) 'merge
	in' intrpvar.h t/base/term.t (@7895..) win32/vdir.h (@8156..)
	thrdvar.h (@8602..) perlapi.c (@8606..)