Add a mailbox interface.

Mailboxes are an IPC mechanism found on several ARM architectures, namely
OMAP4, BCM2385, Sitara, etc.

Reviewed by:	gonzo
Document tip on how to build all kernels quickly.
Correct typo specifying jflags.
Sandbox tcpdump(8) using Capsicum's capability mode and capabilities.
For now, sandboxing is done only if -n option was specified and neither -z nor
-V options were given. Because it is very common to run tcpdump(8) with the -n
option for speed, I decided to commit sandboxing now. To also support
sandboxing when -n option wasn't specified, we need Casper daemon and its
services that are not available in FreeBSD yet.

- Limit file descriptors of a file specified by -r option or files specified
  via -V option to CAP_READ only.

- If neither -r nor -V options were specified, we operate on /dev/bpf.
  Limit its descriptor to CAP_READ and CAP_IOCTL plus limit allowed ioctls to
  BIOCGSTATS only.

- Limit file descriptor of a file specified by -w option to CAP_SEEK and
  CAP_WRITE.

- If either -C or -G options were specified, we open directory containing
  destination file and we limit directory descriptor to CAP_CREATE, CAP_FCNTL,
  CAP_FTRUNCATE, CAP_LOOKUP, CAP_SEEK and CAP_WRITE. Newly opened/created
  files are limited to CAP_SEEK and CAP_WRITE only.

- Enter capability mode if -n option was specified and neither -z nor -V
  options were specified.

Approved by:	delphij, wxs
Sponsored by:	The FreeBSD Foundation
Another warning.
Convert bcm2835_mbox to the new mbox interface.

Reviewed by:	gonzo