Major refactor, very non-atomic change.

 - Remove all command-line arguments except one: -o for offset file.
   If last character in that file is a '/', then the option value
   specifies the directory to store offset files in.

 - Zap MAX and just use MY_PATH_MAX (renamed from MY_MAX_PATH to
   mirror operating system limit name.

 - Use built-in dirname() and basename() functions, instead of rolling
   our own.  Accomodate Solaris and others that change arg to these
   functions.

 - Pull logic for creating offset filename into its own function.

 - Replace (fprintf,exit) idiom with errx and err, depending on if
   errno is set by the failing call.

 - Get rid of -h option.  I plan to write a man page.