sd-dhcp6-client: allow to build large packet

sd-dhcp6-client: allow to build large packet

Previously, the maximum packet size was hardcoded with 512 + size of
the DHCP header. This makes the packet size increased when necessary.
See option_append_hdr().

Previously, all functions which append DHCP options took the head of the
unused area of the packet, and the size of the free area.
However, with this change, the buffer for the whole packet may be
reallocated, hence now they take the head of the packet and the offset
to the free area.

Fixes #24851.