Fix #224: support for cmdline -- finit.fstab=/etc/fstab.secondary

This adds support for bringing up the system with an alternate fstab at
boot.  E.g., when using a primary/secondary setup for boot partitions.

By default /etc/fstab is read, like before, this can now be changed
using configure --with-fstab=/path/to/fstab.primary, which sets the
default that can be overridden using finit.fstab=/etc/fstab.secondary

If mounting, or fsck, fails in any way, Finit calls its own bundled
sulogin, or the system sulogin(8), to let the user handle the issue.
If there is no sulogin available, Finit will try to start up in its
rescue.conf boot mode.

Please note, in either of these rescue modes, use `reboot -f` to get the
system to reboot.  Finit is on pause in the background in rescue mode
and cannot be relied on (since there may not be any writable filesystems
available.).

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

doc/cmdline.md

@@ -15,6 +15,25 @@ The `bool` setting is one of `on, off, true false, 1, 0`.
 * `finit.debug[=bool]`: Enable finit debug.  This is operated
 	independently of the kernel `debug` setting.  New as of Finit v4.
 
+* `finit.fstab=</path/to/etc/fstab.alternative>`: Tell Finit to use an
+  alternate `fstab` to mount the file system from.  Remember, this file
+  must be on the `root=...` file system provided to Finit from the
+  kernel.  By default the built-in fstab is used, which itself defaults
+  to `/etc/fstab`, but can be changed at build time with:
+
+        ./configure --with-fstab=/path/to/fstab
+
+  It is even possible to disable a built-in default using:
+
+        ./configure --without-fstab
+
+  Making `finit.fstab=/path/to/fstab` a *mandatory* command line option.
+  Note, if the command line fstab is missing, Finit falls back to the
+  built-in fstab, and if both are missing, the system treats this as a
+  bad `fsck` and thus calls `sulogin`.  If, in turn, `sulogin` is not
+  available on the system, Finit calls reboot, which is also what will
+  happen when a user exits from `sulogin`.
+
 * `finit.status[=bool]`: Control finit boot progress, including banner.
   (Used to be `finit.show_status`, which works but is deprecated.)