[libsmi] esoteric error level

On the mreview@ops.ietf.org list, I suggested to add a new error level 7 for "esoteric notices". The idea is to change the default error to level 7 for those warnings which the IETF MIB review guidelines document considers to be not important or noise (such as namelength-32 or case-redefinitions).

This means that MIB reviewers following the IETF guidelines can simply run `smilint -c /dev/null -l 6' to get the right amount of warnings produced.

Frank, do you agree with this chance? Is there anyone against introducing level 7 as described above? Let me know quickly so that I can put this in place next week.

/js