hyperfile/sysctl: reject unregisterable sysctl paths (fixes aarch64 boot panic)

[lacraig2]

Problem

Auto-generated pseudofile models scrape binary strings and emit bogus /proc/sys/* entries that get registered as sysctls, e.g.:

/proc/sys/fs/binfmt_misc/WSLInteropos/signal
/proc/sys/kernel/hostname2006
/proc/sys/net/core/somaxconnabi
/proc/sys/none

Registering these is useless at best. At worst it is fatal: for filesystem-backed nodes such as /proc/sys/fs/binfmt_misc, the guest kernel can't create a ctl_table there, and on older kernels (e.g. 4.10) the failed register_sysctl() panics in its cleanup path (drop_sysctl_table → rb_erase), killing init. This reliably crashed aarch64 guests ~5s into boot.

Fix

Reject unregisterable sysctl paths at the registration chokepoint (Sysctl.register_sysctl), with a logged warning instead of queueing them:

• empty paths,
• paths containing an empty component (//),
• the fs/binfmt_misc subtree — it's a mounted filesystem, not a sysctl, so modeling it as one is always wrong.

plugins.sysctl WARNING  Skipping sysctl registration for
  'fs/binfmt_misc/WSLInteropos/signal':
  /proc/sys/fs/binfmt_misc is a mounted filesystem, not a sysctl

Testing

Ran the GL.iNet Beryl AX (GL-MT3000, aarch64/4.10) firmware under Penguin:

• Before: kernel panic at ~5s (Attempted to kill init!), 99-line console, no services.
• After: the guard skips the bad entry (warning above), no panic, and the guest boots normally (≈300-line console). An equivalent run with the offending pseudofiles removed brings up the full service set (uhttpd, dnsmasq, samba, avahi), confirming the guest is healthy once the bad registration is avoided.

Companion

rehosting/igloo_driver#76 hardens the kernel module so a doomed registration can never panic the guest (it refuses to create a sysctl under a non-existent parent directory on old kernels). This PR is the upstream half: don't emit the bogus models in the first place. Defense in depth — either alone prevents the panic.

Note: the deeper aarch64 fidelity issue (Penguin ships a 4.10 kernel for aarch64 while the firmware's own modules target 5.4) is separate and tracked elsewhere; this PR only addresses the sysctl-registration crash.