scripts: misc help and auto-complete fixes

- don't list samples in the main help screen
- improve the samples listing in list-samples
- don't document the 'config' action, it's long dead
- document the 'V' environment variable
- improve on START, STOP and PREFIX environment variables
- add PREFIX and V to autocomplete
- advertise auto-complete at install time

 --- binutils/ld/ldmain.c

 +++ binutils/ld/ldmain.c

 @@ -273,2 +273,5 @@ main (int argc, char **argv)

    link_info.emit_hash = TRUE;

 +#ifndef __mips__

 +  link_info.emit_gnu_hash = TRUE;

 +#endif

    link_info.callbacks = &link_callbacks;