Add a CRC hash for the name of each mod item (unittype, objecttype, etc).

This is for network support, so we can send the CRC32 through the network
instead of the full name.

The CRC function is Copyright 1986 Gary S. Brown, used with permission.

Source URL:
http://www.opensource.apple.com/source/xnu/xnu-1456.1.26/bsd/libkern/crc32.c