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

 @@ -293,2 +293,3 @@ main (int argc, char **argv)

    link_info.combreloc = TRUE;

 +  link_info.relro = TRUE;

    link_info.strip_discarded = TRUE;