lib: use "union prefixconstptr" in table code

lib: use "union prefixconstptr" in table code

This allows passing struct prefix_{ipv4,ipv6,evpn} * in addition to
struct prefix * without an extra cast (since the union uses the gcc
transparent-union extension present in all compilers that we support.)

Also applies some "const" while we're at it.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>