Use proper ordering in EXAMPLES section in growfs(8).

MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Document growfs(8) feature apparently nobody knows about.

MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Include a random device.
* Add a random device
* Use the same rootdev path as MALTA, so the same emulator setups
  can be used with minimal fuss.
Add example on how to use gpart before growfs.  While here, reorder examples
so that the simplest one comes first.

MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Uniformly refer to a file system as "file system".

MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
cpudep_ap_early_bootstrap() takes no arguments, so no need to give it one.

MFC after:	3 weeks
Set the current vnet inside the ioctl handler for ipfilter.

Without this fix, the vnet was NULL and would crash.
This fix is similar to what was done inside the ioctl handler for PF.

Tested by:
   (1) Boot a kernel with "options VIMAGE" enabled
   (2) Type:

   echo "map lo0 from 10.0.0.0/24 to ! 10.0.0.0/24 -> 127.0.0.1/32" > /etc/ipnat.rules ; service ipnat onerestart

PR: 176992
Differential Revision: https://reviews.freebsd.org/D1191
Reviewed by: cy
MFp4: @179066

Add page which describes VNET network stack virtualization infrastructure.

Submitted by: bz
Sponsored by: The FreeBSD Foundation
Sponsored by: CK Software GmbH