This adds definition of a set of known route option attributes to
libnm-core and helper functions.

nm_ip_route_attribute_validate() performs the validation of the
attribute type and, in case of a formatted string attribute, of its
content.

nm_ip_route_get_variant_attribute_spec() returns the attribute format
specifier to be passed to nm_utils_parse_variant_attributes(). Since
at the moment NMIPRoute is the only user of NMVariantAttributeSpec and
the type is opaque to users of the library, the struct is extended to
carry some other data useful for validation.