Provide m_catpkt(), a wrapper around m_cat() that deals with M_PKTHDR mbufs.

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.
Improve r265338. When inserting mbufs into TCP reassembly queue,
try to collapse adjacent pieces using m_catpkt(). In best case
scenario it copies data and frees mbufs, making mbuf exhaustion
attack harder.

Suggested by:		Jonathan Looney <jonlooney gmail.com>
Security:		Hardens against remote mbuf exhaustion attack.
Sponsored by:		Netflix
Sponsored by:		Nginx, Inc.