lib: create `frr_daemon_state_{load,save}`

lib: create `frr_daemon_state_{load,save}`

These functions load daemon-specific persistent state from
`/var/lib/frr` and supersede open-coded variants of similar calls in
ospfd, ospf6d and isisd to save GR state and/or sequence numbers.

Unlike the open-coded variants, the save call correctly `fsync()`s the
saved data to ensure disk contents are consistent.

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