Use symbol for internal key.

To separate items for internal use in the RESOLVE_IPNETS__IPFAMILIES
hash from flags passed in as parameters to resolve_ipnets(), use a
symbol instead of a string as the marker for unspecified IP family.
That way, there is also no risk of it being matched against a family
marker in the netspec either (i.e, the "ipv[46]:" prefix).

This was originally commit aff28d82e703 in nsc-puppet-utils.