Avoid assuming that type key_t is 32 bits, since it reportedly isn't

Avoid assuming that type key_t is 32 bits, since it reportedly isn't
on 64-bit Solaris.  Use a non-system-dependent datatype for UsedShmemSegID,
namely unsigned long (which we were already assuming could hold a shmem
key anyway, cf RecordSharedMemoryInLockFile).