samples: sockets: big_http_download: Update for DHCPv4 support.

Enable CONFIG_NET_DHCPV4, while keeping static IP configuration,
which allows the app to work both with static IP (e.g. when
connected to a workstation without DHCP server) and with DHCP (e.g.
when connected to a router). When using DHCP, getting an address
may take some time, during which getaddrinfo() may fail due to a
timeout. So, add retries for this call.

Tested with qemu_x86 (non-DHCP) and frdm_k64f (both DHCP and
non-DHCP).

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>