sd-dhcp-client: correct dhcpv4 renew/rebind retransmit timeouts

sd-dhcp-client: correct dhcpv4 renew/rebind retransmit timeouts

Use the request timeout algorithm specified in RFC2131 section 4.4.5 for
handling timed out RENEW and REBIND requests.

This changes behavior, as previously only 2 RENEW and 2 REBIND requests
were sent, no matter how long the lease lifetime. Now, requests are
send according to the RFC, which results in starting with a timeout
of 1/2 the t1 or t2 period, and halving the timeout for each retry
down to a minimum of 60 seconds.

Fixes: #17909