[lwip-members] Pbufs

[Kieran Mansley]

Thanks for doing the pbuf_realloc thing - something I've been meaning to
do for some time!

I've made a number of other additions to pbuf code in the last few weeks.
I added a PBUF_HEADER type which simply allocates a pbuf large enough for
a TCP/IP header from a new memp pool.  Allocating this size of pbuf is a
common operation when applications are passing in their own data (in the
way that has lead to the PBUF_REF suggestion), so it made sense to me to
make this fast.  It's also particularly useful for me as the network
delivers the packet headers and payloads separately.

If anyone else is interested I can merge this into the main tree without
too much trouble, but thought I'd ask for comments first.

Kieran