Move {show,set,clear}_setting() to {show,set,clear}_named_setting().

Move {show,set,clear}_setting() to {show,set,clear}_named_setting().

Introduce new {show,set,clear}_setting() that take a struct setting *
rather than a const char *.

set_setting() handles calling clear_setting() when appropriate, so that
individual setting types don't have to check for empty strings.