odhcpd: multiple DHCPv4 improvements (thx Christian Mehlis)

Signed-off-by: Steven Barth <steven@midlink.org>

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@42154 3c298f89-4303-0410-b956-a3cf2f4a3e73