Hello,

This follows through a suggestion (from Fernando?) from long ago.  It 
replaces add_show_from_set (add_set_boolean_cmd(...), ...); with a more 
complete add_setshow_boolean_cmd().

Since the function add_show_from_set() pulls a decidedly i18n unfriendly 
trick, I've not emulated that.  Instead the setshow function takes 
separate SET and SHOW documentation strings.

committed,
Andrew

(I'm going to ari add_show_from_set() as well)