Add trace-style DNR searching capabilities.

Add trace-style DNR searching capabilities.

src/chanserv.c (msgtab): Separate DNR info strings for easier translation.
    Add DNR bad-action and search results strings.
  (dnr_print_func): New function to reduce code duplication.
  (chanserv_show_dnrs): Use dnr_print_func() instead of doing the display
    in this function.
  (send_dnrs): Likewise.
  (cmd_allowregister): Simplify checks for DNR existence and removal.
  (struct dnr_search): New type.
  (dnr_search_matches): New function.
  (dnr_search_create): New function.
  (dnr_search_func): New typedef.
  (dnr_search): New function.
  (dnr_remove_func): New function.
  (cmd_dnrsearch): New function.
  (init_chanserv): Register dnrsearch and its subcommands.

src/chanserv.help: Document DNRSEARCH command.