make snoopy dissect gre.
there doesn't seem to be a registry that keeps track of protocol numbers used in gre.
the current ones (used in in the mux structure) come from what was in the old gre.c and an older gre rfc (with all too-ancient-looking protocols removed (criteria i used is that they had to be present in ip.c).  the eopol, dot1q, llc were used in other software.