Support symbolic links.

The following changes were made to support symbolic links:
* Provide a function to compute the CRC32 of a string
* Use lstat instead of stat so that printnode will detect symbolic links instead of following them
* Have printnode print the size, CRC and contents of the link
* Have mkbom store symlinks in the BOM
* Remove the symlink restriction from the BUGS section of the ls4mkbom and mkbom man pages