Simplify G-line code somewhat. Generate better output for ?gtrace print.

Simplify G-line code somewhat.  Generate better output for ?gtrace print.

A hub with a negative clock skew will cause adverse results (like the
issued or lastmod times being in the network's future).  That will be
fixed in ircu.

src/gline.c (gline_for_p): Rewrite to just compare the G-line pointer.
  (gline_add): Use that instead.  Move "issued" back in time if the
    lastmod time is earlier than we know the G-line was issued.
src/opserv.c (OSMSG_GTRACE_FOREVER): New format string.
  (OSMSG_GTRACE_EXPIRED): Likewise.
  (gtrace_print_func): Use the appropriate OSMSG_GTRACE_* string.