Rework the packet.c code for decoding and assembling DHCP messages.

Rework the packet.c code for decoding and assembling DHCP messages.

This code suffered from many years of incremental improvements, fix it to:
- verify available buffer space in all cases and don't just trust the caller
- have an API where you always pass the full buffer length and absolute offset
- use consistent types for lengths and buffer offsets (int vs. size_t, ssize_t)
- don't just ignore errors, return and fail when something is wrong

OK rzalamena@