Add support for a rescue (recovery) mode from kernel cmdline

This patch adds a rescue (revovery) mode to Finit.  Simply add `rescue`
to the kernel cmdline at boot and the following steps at bootstrap will
be skipped:

- Load of services/run/task/etc in /etc/finit.conf + /etc/finit.d/*.conf
- fsck of filesystems in /etc/fstab
- Remount of / to read-write
- Mount of all filesystems in /etc/fstab
- swapon
- A few plugin hooks:
  - HOOK_ROOTFS_UP
  - HOOK_MOUNT_ERROR
  - HOOK_BASEFS_UP
- `runparts DIR`
- /etc/rc.local

Instead of loading /etc/finit.conf and /etc/finit.d/*.conf Finit loads
the file /lib/finit/rescue.conf, which can be overridden by the operator
if needed.  If this file is removed (or lost), Finit falls back to start
a simple root shell, without any login.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

doc/TODO.md

@@ -21,9 +21,6 @@ can also be used.
 General
 -------
 
-* Add support for `rescue` on the kernel command line, start rescue mode
-  if enabled with `configure` script.  Could mean skip `/etc/finit.conf`
-  et al and jump straight to a single getty login on console.
 * Add support for timed shutdown in Finit, including cancelled shutdown.
 * SysV init and systemd use SIGUSR1 to restart their FIFO/D-Bus.  Add
   API restart to SIGHUP callback for the same functionality in Finit.