Refactor dhcp_utils for more flexible uses.

Separate out fetching DHCP results from starting the daemon
(for either discover or renew).

Renamed dhcp_do_request() to dhcp_start().
Renamed dhcp_do_request_renew() to dhcp_start_renew().
Added new dhcp_get_results().

Callers of the prior versions of dhcp_do_request* now separately
call dhcp_get_results().

Bug: 19422416
Change-Id: I05447853d444015e6014661dda1bb177b58fe463