(_cmalloc): Reorganize to accomodate muto locking.
(_cfree): Use muto lock to avoid multi-thread problems.
* cygheap.h (incygheap): Just use cygheap_max to find upper cygwin heap bounds.
* dcrt0.cc (dll_crt0_1): Reinstitute cygheap_init call.
* path.cc (getcwd): Just return cwdstuff::get result, allowing correct handling
of negative length.
(cwdstuff::get): Malloc a buffer if one is not available.
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