Fix DNR expiration and expiration-based DNR searches.

src/chanserv.c (expire_dnrs): Use the usual trick for iterating over a dict
    while we might want to delete elements from it.
  (dnr_search_matches): Simplify the min_expires test.  If max_expires is
    set, fail if the DNR never expires or it expires later.
  (dnr_search_create): Initialize max_expires to 0, not INT_MAX.