sd-dhcp-client: make client initially in stopped state

sd-dhcp-client: make client initially in stopped state

Previously, DHCP_STATE_STOPPED and DHCP_STATE_INIT are both handled as
not-running. Moreover, previously after sd_dhcp_client_start() is
called, the client still in INIT state (and thus handled as not-running)
even if its internal timer event sources are initialized.

Let's make only STOPPED state handled as not-running, and make the
client initially in STOPPED state.

Prompted by #28896.