In VM and cloud environments it is often possible to enlarge virtual
disks; this can be useful, for example, if a system is launched with a
small root disk and it later becomes clear that more space is needed.
On kernels which support run-time resizing of disks (for NVMe, this was
added in November 2025; some other disk types have supported this for
longer) a SIZECHANGE notification is sent to userland via devd.
Add a "nostart" rc.d script (runnable manually but not automatically at
boot time) and a devd script which invokes it when a notification
arrives. The rc.d script enlarges the "final partition" on partitioned
geoms, or the UFS filesystem or zpool device when triggered on a disk
containing either of those.
MFC after: 2 weeks
Relnotes: Disk partitions and filesystems can be enlarged
automatically when disks grow by setting growfs_postboot_enable=YES in /etc/rc.conf.
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