(shm_unlink): Export.
* syscalls.cc (shm_open): New function.
(shm_unlink): New function.
* sysconf.cc (sca): Set value of _SC_SHARED_MEMORY_OBJECTS to
_POSIX_SHARED_MEMORY_OBJECTS.
* include/cygwin/version.h: Bump API minor number.
* include/sys/mman.h (shm_open): Add prototype.
(shm_unlink): Ditto.