This change adds support for at least FreeBSD 8.4.0, but other old
versions might work as well.

Included changes:

- support for pmap structure without pm_cr3 member (before 9.3)
- support for filedesc structure with fd_ofiles member of type
  `struct file **` (before 9.2)
- support for cdev structure with si_name member of type `char *`
  (before 9.1)

Adds an inital version of freebsd_tcpconns command, which allows to list
active TCP connections.

This plugin is based on the following publication:

Bond, Elyse, "Creating Volatility Support for FreeBSD" (2015).
University of New Orleans Theses and Dissertations. 2033.

Also includes a check for missing tcbinfo symbol in tcpconns command.

Define which FreeBSD versions introduced 2 structures that are not
available in 8.4.