net: dhcpv4: Move DHCPv4 files to lib directory

As discussed during DHCPv4 server integration, group DHCPv4 client files
with DHCPv4 server in a single lib directory.

Renamed internal "dhcpv4.h" header to "dhcpv4_internal.h" so that it's
not confused with the public "dhcpv4.h" header.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>