Improvements to the MDXFileChunk() template function:

• Remove unneeded fstat()/lseek() calls.
• Return NULL and set errno to EINVAL on negative length.
• Fix small style problems and expand variable names.

After this change, it is possible to use this code for some irregular
files. For example, 'md5 /dev/md0' should now succeed.

Differential Revision: https://reviews.freebsd.org/D4748
Suggested by: bde
Reviewed by: bde, allanjude, delphij