Implement switch_root functionality allowing Finit to serve as the init

in an initramfs, then transition to the real root filesystem.  Useful
for systems requiring early boot tasks like LUKS unlock, LVM activation,
or network boot before mounting the real root.

Adds INIT_CMD_SWITCH_ROOT API command, `initctl switch-root` subcommand,
and HOOK_SWITCH_ROOT plugin hook point.  The implementation gracefully
stops services, moves virtual filesystems (/dev, /proc, /sys, /run) to
the new root, deletes initramfs contents to free memory, then execs the
new init as PID 1.

See GitHub Discussion #292 for background.

doc/plugins.md

@@ -205,6 +205,16 @@ hook points:
   new runlevel have been been stopped.  When the hook has completed,
   Finit continues to start all services in the new runlevel.
 
+### Switch Root Hooks
+
+* `HOOK_SWITCH_ROOT`, `hook/sys/switchroot`: Called when
+  `initctl switch-root` is issued, before the transition begins.  Use
+  this hook to save state, unmount initramfs-only filesystems, or perform
+  cleanup before switching to the new root.  Only runs when Finit is
+  operating as PID 1 in an initramfs.
+
+  See the [Switch Root](switchroot.md) section for more information.
+
 ### Shutdown Hooks
 
 * `HOOK_NETWORK_DN`, `hook/net/down`: Called right after having changed