Integrate maintperl changes #12268 and #12669;

Integrate maintperl changes #12268 and #12669;

final touches to the audit for statics and thread-unsafe code
* make DB_File, ODBM_File thread-safe
* remove unnecessary/dangerous statics and protect others
  from not getting accidentally enabled under threaded perls

windows support functions get_childdir() et al aren't exported
correctly under vanilla build

Testing under win32 appreciated since changes there had
to be manually merged and I cannot test how badly did I do.

p4raw-link: @12268 on //depot/perlio: bb407f0b8769c638c05e60ebfd157a1e676a6c22

p4raw-id: //depot/perl@12678
p4raw-integrated: from //depot/maint-5.6/perl@12677 'copy in'
	win32/vmem.h (@5902..) 'merge in' ext/DB_File/DB_File.xs
	(@8693..) win32/win32iop.h (@8917..) ext/ODBM_File/ODBM_File.xs
	(@8995..) iperlsys.h (@9154..) scope.c (@9584..) makedef.pl
	(@11425..) gv.c (@12026..) op.c (@12145..) util.c (@12220..)
	toke.c (@12550..) ext/B/B.xs ext/File/Glob/Glob.xs
	ext/Opcode/Opcode.xs ext/re/re.xs (@12653..) mg.c win32/win32.c
	(@12668..)