Improve IPv6 RA support by allowing dhcpcd to manage the address and routes

Improve IPv6 RA support by allowing dhcpcd to manage the address and routes
instead of the kernel. dhcpcd will only do this if RA is disabled in the kernel
or dhcpcd has been instructed to do this via dhcpcd.conf(5) ipv6ra_own and
ipv6ra_own_default directives.

Send and process IPv6 Neighbor Solicitions and Adverts to prove router
reachability. If a router cannot be reached in this way then it is expired.

When debugging, all ND messages are displayed which will create a lot of log
spam.

To ease packaging, ./configure now accepts LDFLAGS and --enable-static.