growfs script: add swap partition as well as growing root

Add the ability to create a swap partition in the course of growing
the root file system on first boot, enabling by default. The default
rules are: add swap if the disk is at least 15 GB (decimal), and the
existing root is less than 40% of the disk. The default size is 10%
of the disk, but is limited by the memory size. The limit is twice
memory size up to 4 GB, 8 GB up to 8 GB memory, and memory size over
8 GB memory. Swap size is clamped at vm.swap_maxpages/2 as well.
The new swap partition is labeled as "growfs_swap".

The default behavior can be overridden by setting growfs_swap_size in
/etc/rc.conf or in the kernel environment, with kenv taking priority.
A value of 0 inhibits the addition of swap, an empty value specifies
the default, and other values indicate a swap size in bytes.

By default, addition of swap is inhibited if a swap partition is found
in the output of the sysctl kern.geom.conftxt before the current root
partition, usually meaning that there is another disk present.
Swap space is not added if one is already present in /etc/fstab.

The root partition is read-only when growfs runs, so /etc/fstab can
not be modified. That step is handled by a new growfs_fstab script,
added in a separate commit. Set the value "growfs_swap_pdev" in kenv
to indicate that this should be done, as well as for internal use.

There is optional verbose output meant for debugging; it can only be
enabled by modifying the script (in two places, for sh and awk).
This should be removed before release, after testing on -current.

Discussed with: cperciva
Reviewed by: imp (previous version)
Differential Revision: https://reviews.freebsd.org/D37462