Fix G-line handling and ?gtrace time display.

Fix G-line handling and ?gtrace time display.

src/gline.c (gline_comparator): Use lifetime rather than expiration time to
    sort G-lines.
  (gline_add): Only update expiration time and reason if the new
     modification time is newer than the previous one.  Never update
     issuer (ircu doesn't track it).
src/opserv.c (gtrace_print_func): Display G-lines with no expiration time
  correctly.