functestlib: make try_dhcp_client_safe skip DHCP when IP already exists

- In try_dhcp_client_safe(), detect a valid (non link-local) IPv4 early and
  return success without launching DHCP again.
- When systemctl is available and NetworkManager/systemd-networkd is active,
  wait for IP instead of running udhcpc to avoid fighting the manager.
- Keep the udhcpc safe-script behavior for minimal images while improving
  debug visibility around DHCP decisions and outcomes.

Signed-off-by: Srikanth Muppandam <smuppand@qti.qualcomm.com>

Author: smuppand