Fix DHCP lease time parsing.

Currently we treat a lease time larger than 2**31-1 as a negative
value, which causes DhcpClient to attempt to renew its IP address
constantly. Fix this by properly handling large and infinite
lifetimes, and while we're at it, impose a minimum lease time of
60 seconds.

Bug: 21352084
Change-Id: If62c9efeffad6222e2fe0c110f77d0e4c70de96d