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
Differential Revision: https://reviews.freebsd.org/D9099