crc_table is made using a four-byte integer (when that can be
determined). However get_crc_table() returned a pointer to an
unsigned long, which could be eight bytes. This fixes that by
creating a new z_crc_t type for the crc_table.
This type is also used for the BYFOUR crc calculations that depend
on a four-byte type. The four-byte type can now be determined by
./configure, which also solves a problem where ./configure --solo
would never use BYFOUR. No the Z_U4 #define indicates that four-
byte integer was found either by ./configure or by zconf.h.
