MFC r311744,r312328,r312329,r312330:

r311744:

Document bsd.snmpmod.mk from a high-level

r312328:

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.

r312329:

Remove ad hoc smilint targets made standard in bsd.snmpmod.mk in r312328

r312330:

Add smilint target to subdir targets so "make smilint" here will run
the smilint target in subdirs

While here, convert a path that's .CURDIR relative to SRCTOP