Clear content of keyfiles loaded by the loader after processing them.

Pointed out by:	rwatson
MFC after:	1 week
Clear some more places with potentially sensitive data.

MFC after:	1 week
MFp4 @1189139:

Get rid of the msg_peek() function, which has a problem.  If there was less
data in the socket buffer than requested by the caller, the function would busy
loop, as select(2) will always return immediately.

We can just receive nvlhdr now, because some time ago we splitted receive of
data from the receive of descriptors.

MFC after:	1 week
MFp4 @1189141:

Change casperd's zygote process title.

MFC after:	1 week
Make use of casperd's system.dns service when running without the -n option.
Now tcpdump(8) is sandboxed even if DNS resolution is required.

Sponsored by:	The FreeBSD Foundation
Add Casper support.

Sponsored by:	The FreeBSD Foundation
Make use of Casper's system.pwd and system.grp services when the -r option
is given to convert uids and gids to user names and group names even when
running in capability mode sandbox.

While here log on stderr when we successfully enter the sandbox.

Sponsored by:	The FreeBSD Foundation
Forgot to regenerate after r257736.
Allow for pselect(2) in capability mode.

Noticed by:	David Drysdale <drysdale@google.com>
Regenerate after r259436.
Fix syscalls that can be loaded as kernel modules - they were not given
the flag allowing to call them from capability mode sandbox.

Noticed by:	David Drysdale <drysdale@google.com>
Regenerate after r259438.