Use VirtualAlloc() more flexibly when using it to mimic UNIX's sbrk().

Use VirtualAlloc() more flexibly when using it to mimic UNIX's sbrk().
CC: perl-win32-porters@listserv.ActiveState.com
Message-ID: <40F6B295.8010804@uk.radan.com>

Assumes perl's malloc can now handle non-contiguous memory (believed
to be true).
Does not address threading issues.

"The attached patch (against blead) makes sbrk() initially try to
extend the existing block of memory exactly as it currently does, but
to not  fail immediately if it can't -- it now frees up that part of
whatever it  had previously reserved+committed which hadn't actually
been used yet,  resets all its static variables and basically starts
anew."

p4raw-id: //depot/perl@23128