The TCPPCAP debugging feature caches recently-used mbufs for use in
debugging TCP connections. This commit provides a mechanism to free those
mbufs when the system is under memory pressure.

Because this will result in lost debugging information, the behavior is
controllable by a sysctl. The default setting is to free the mbufs.

Reviewed by: gnn
Approved by: re (gjb)
Differential Revision: https://reviews.freebsd.org/D6931
Input from: novice_techie.com