Add a make target (smilint) for running smilint tool against BMIBS
Running smilint against MIB definitions is useful in finding
functional problems with MIB definitions/descriptions.
This is inspired by the smilint targets defined in
usr.sbin/bsnmpd/modules/{snmp_hostres,snmp_mibII}/Makefile
Document all of the variables that are involved in running the
smilint target, as well as all of the prerequisites to running
it.
MFC after: 1 week