networkd: set route protocol

All routes added by networkd are currently set RTPROT_BOOT, which according
to the kernel means "Route installed during boot" (rtnetlink.h).  But this
is not always the case as networkd changes routing after boot too.  Since
the kernel gives more detailed protocols, use them.

With this patch, user-configured static routes now use RTPROT_STATIC (which
they are) and DHCP routes use RTPROT_DHCP.  There is no define for IPv4LL
yet, so those are installed as RTPROT_STATIC (though perhaps RTPROT_RA is
better?).

[tomegun: fixup
src/network/networkd-link.c:972:33: error: too few arguments to function 'route_new_dynamic']
dhcp-network: remove unused DHCP6_STATE_RS

Probably a left-over from when router solicitations were
requested in the DHCP6 code.  But since they are now separate,
this state is no longer needed.