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/initctl.md

@@ -65,6 +65,7 @@ Commands:
   halt                      Halt system
   poweroff                  Halt and power off system
   suspend                   Suspend system
+  switch-root NEWROOT [INIT]  Switch to new root filesystem (initramfs only)
 
   utmp     show             Raw dump of UTMP/WTMP db
 ```
@@ -143,3 +144,24 @@ Apr  8 12:37:46 alpine authpriv.info dropbear[2300]: Exit (root) from <192.168.1
 Apr  8 15:02:11 alpine authpriv.info dropbear[2634]: Child connection from 192.168.121.1:48576
 Apr  8 15:02:12 alpine authpriv.notice dropbear[2634]: Password auth succeeded for 'root' from 192.168.121.1:48576
 ```
+
+
+Switch Root
+-----------
+
+The `switch-root` command is used when running Finit in an initramfs to
+transition to the real root filesystem.  This is similar to the standalone
+`switch_root(8)` utility but integrated into Finit.
+
+```
+initctl switch-root /mnt/root [/sbin/init]
+```
+
+Requirements:
+
+- Must be run during runlevel S (bootstrap) or runlevel 1
+- The new root must be a mount point (different device than /)
+- Can only be used when Finit is running as PID 1 in an initramfs
+
+For complete documentation and usage examples, see the dedicated
+[Switch Root](switchroot.md) section.