Fix truncation when ssize_t is larger than MAX_INT

Casting to int truncates size on some platforms, resulting swab not
copying all the data. Cast len to size_t to avoid right shifting a
signed value: we know here it's > 0, so we can safely cast it w/o losing
precision.

In addition, be more careful with signedness of char pointers and
temporaries. Downgrade tmp from unsigned long to unsigned char since
we're only reading and writing characters.

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/516