sd-dhcp-client: enter the SELECTING state before sending DHCPDISCOVER

sd-dhcp-client: enter the SELECTING state before sending DHCPDISCOVER

Similarly, enter the REBOOTING state before sending DHCPREQUEST on reboot.

Also, this makes DHCPREQUEST message is sent several times also in REBOOTING
state. Previously, we wait about 4 seconds after DHCPREQUEST on reboot, and
entered the init state if no response. Now we wait 1 second after the first
DHCPREQUEST, resend another DHCPREQUEST, wait 2 seconds, then enter the
init state if no response. So, even in the worst case, we have slight
speed up.