shared/options: add new helper option_parser_get_arg

shared/options: add new helper option_parser_get_arg

option_parser_next_arg() is renamed to option_parser_peek_next_arg()
to match option_parser_consume_next_arg().

A new helper is added option_parser_get_arg(…, n). It is a common pattern
to only need a single arg, and getting an array and extracting a single
item from it is too verbose.