rc+devd: Add growfs_postboot

[cperciva]

Summary

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.

Sponsored by: Amazon

[ziaee]

This is all I can find so far, but I didn't test it yet.

libexec/rc/rc.d/growfs_postboot

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Please consider using this format for consistency with style guides and C style comments.

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share/man/man7/growfs_postboot.7

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Please consider using this format recommended by out style guides for consistency with C comments.

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No change requested here, just note:

1. both this and growfs(7) really shouldn't exist and should be part of rc.conf(5) for discoverability
2. failing that, system operation commands do belong in section 8 per intro(8)
3. failing that, we might as well match growfs(7) like the patch is currently doing... for now

Hopefully one day we can rewrite growfs(7), growfs_fstab(7), and this manual into a combination of rc.conf(5) and improved comments on the scripts themselves.

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Macros inside list widths are cursed

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Since you wrote everything, optionally consider using traditional style. Alternatively, it may make sense to append .An -nosplit after line 83 to change the line wrapping for when the AUTHORS section contains prose.