lib: Add zapi_route function.

lib: Add zapi_route function.

Allow routing protocols to call one function to add/delete
routes into zebra.  Future commits will start adding
this code to individual routing protocols.

Why are we doing this?  Well the zapi_ipv[4|6]_route functions
are fundamentally broken in their ability to pass down anything
but NEXTHOP_TYPE_IFINDEX or NEXTHOP_TYPE_IPV[4|6] and we need
the ability to pass down a bit more information.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>