snmp: workaround unknown value for RONLY

This happens when Net-SNMP has been compiled with
NETSNMP_NO_LEGACY_DEFINITIONS defined. For example, on Mac OS X. This
seems a good way to deprecate an API still widely used just because it
is not used internally... <grin>
osx: update Homebrew formula with JSON and SNMP support
osx: don't include "head" bits in homebrew formula

We will add them back when they work.