6lowpan: iphc: rename hc06_ptr pointer to hc_ptr

The hc06_ptr pointer variable stands for header compression draft-06. We
are mostly rfc complaint. This patch rename the variable to normal hc_ptr.
6lowpan: iphc: use sizeof in udp uncompression
6lowpan: iphc: cleanup use of lowpan_fetch_skb

We introduced the lowpan_fetch_skb function in some previous patches for
6lowpan to have a generic fetch function. This patch drops the old
function and use the generic lowpan_fetch_skb one.
6lowpan: iphc: cleanup use of lowpan_push_hc_data

This patch uses the lowpan_push_hc_data functions in several places
where we can use it. The lowpan_push_hc_data was introduced in some
previous patches.
6lowpan: iphc: use ipv6 api to check address scope

This patch removes the own implementation to check of link-layer,
broadcast and any address type and use the IPv6 api for that.