dhcpv4: improve pool var naming

dhcpv4: improve pool var naming

The uci cfg options might have somewhat confusing names, but
that doesn't mean we have to perpetuate those names throughout the
source.

Also, move the checking of whether the pool parameters from when
we are setting up a pool to where the cfg variables are actually set
from config.

Finally, there's no need to store the pool start and end as "struct
in_addr" in struct interface, because the start and end actually aren't
IPv4 addresses.

Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/320
Signed-off-by: Álvaro Fernández Rojas <[email protected]>