MFstable/11 r310990:

MFstable/11 r310990:

MFC r310498,r310591:

r310498:

Allow SNMPv3 authNoPriv and noAuthNoPriv protocols to discover snmpEngineTime
as discussed in RFC-5343

This fixes interoperability with net-snmp.

Tested with the following invocations of snmpwalk (from net-snmp):

- noAuthNoPriv:

  % snmpwalk -v 3 -n '' -u public localhost snmpEngineTime

- authNoPriv:

  % snmpwalk -v 3 -n '' -u bsnmp -A bsnmptest -l authNoPriv -a sha localhost \
    localhost snmpEngineTime

- authPriv:

  % snmpwalk -v 3 -n '' -u bsnmp -A bsnmptest -l authPriv -a sha -x des \
    -X bsnmptest localhost snmpEngineTime

Obtained from:	Isilon OneFS (5ec6d772cacbc, with minor tweaks)

r310591:

Update engine time using update_snmpd_engine_time(..)

git-svn-id: svn://svn.freebsd.org/base/stable/10@310991 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f