use enum to specify order and indexes of iov struct.

This helps to avoid brainfuck index calculation errors when adding
features.

On my build machine (arch linux x86_64), the stripped binary has exactly
the same size, so this patch should produce the same binary, but
increase code maintainability.
Merge pull request #16 from kaspar030/use_enum_for_iov

use enum to specify order and indexes of iov struct in dhcp_send().