Tweak DNR expiration code for aesthetics.

Tweak DNR expiration code for aesthetics.

src/chanserv.c (CSMSG_DNR_INFO_SET): Add missing "ago".
  (chanserv_find_dnrs): Add "max" argument.  Revise to remove expired
    DNRs as they are found.
  (chanserv_show_dnrs): Exploit the above.
  (chanserv_is_dnr): Use chanserv_find_dnrs(), removing lots of nearly
    duplicated code.
  (send_dnrs): New function to send unexpired DNRs from one dict.
  (cmd_noregister): Use send_dnrs() to merge duplicated code.
  (write_dnrs_helper): Remove expired DNRs here.