r5456: Increase limit of mapped SIDS to 0x5000, which is what 2k and later do.

r5456: Increase limit of mapped SIDS to 0x5000, which is what 2k and later do.
NT has no limit.  We still don't respond the way 2k would to requests
larger, which is to actually allocate the memory and send back the entire
response, plus a return status of NT_STATUS_NONE_MAPPED.  Still looking
into ways of doing this without crashing windows.