This change adds public KPI to work with routes using pre-created nexthops, instead of using data from addrinfo structures. These functions will be later used for adding/deleting kernel-originated routes and upcoming netlink protocol.

As a part of providing this KPI, low-level route addition code has been reworked to provide more control over route creation or change.
Specifically, a number of operation flags (RTM_F_<CREATE|EXCL|REPLACE|APPEND>) have been added, defining the desired behaviour the the route already exists (or not exists). This change required some changes in the multipath addition code, resulting in moving this code to route_ctl.c, rendering mpath_ctl.c empty.