f3write.h2w: add support to all f3write's commands

The previous version of f3write.h2w didn't recognize
parameters --start-at= and --end-at=.

The new code of f3write.h2w also avoids overwritting
the environment variable PATH.
Add script log-f3wr

This script runs f3write and f3read, and records their output into
a log file.