net-mgmt/net-snmp: Print a warning if config files aren't readable

Motivated by a bug report from dvl@. In his case, snmpd was able to
read the configuration file initially, but failed to reload it upon
receiving SIGHUP during log rotation.

Approved by: zi
Sponsored by: Stormshield
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D45343