check CRC

This release adds several enhancements, the most important of which is
the ability to link against the CRC-32 code in zlib 1.2.1 and later.
This results in an impressive 80% speed improvement over check's native
CRC code, which is basically the same as that found in older versions
of zlib. This release also optimizes the CRC buffer size and adds a -t
option to print the time and date before and after each file (useful
for estimating the completion time of multigigabyte backups and DVD
archives).

Error checking for unreadable files has been fixed (whether due to disk/disc errors or insufficient permissions). The behavior on errors has been changed to continue with the next file (if any). A final summary of the total number of errors has been added.

This version improves option-parsing to support concatenation (e.g., "-acv" as well as "-a -c -v") and corrects an omission in the usage info.