guestfish: write --help to stdout, use gnulib's progname module

* fish/fish.c: Include "progname.h".
(main): Call set_program_name to initialize.
Don't hard-code guestfish everywhere.  Use program_name.
However, be careful when modifying argv[0], since it is used
in the hopes that it is an absolute file name.
(usage): Don't spew all of --help for a mis-typed option.
Split long lines.