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>

man/finit.8

@@ -13,6 +13,7 @@
 .Nm /sbin/finit
 .Op --
 .Op finit.debug
+.Op finit.fstab=/etc/fstab.alt
 .Op finit.status=<on,off>
 .Op finit.status_style=<classic,modern>
 .Op rescue | recover
@@ -77,6 +78,43 @@ Enable Finit debug messages on system console and log.  Sometimes useful
 when doing board bringup.  Before the system log daemon has started,
 Finit use the kernel ring buffer,
 .Pa /dev/kmsg .
+.It Cm finit.fstab=/path/to/fstab.alt
+Tell
+.Nm
+to use an alternate
+.Cm fstab
+to mount the file system from.  Remember, this file must be on the
+.Cm root=...
+file system provided to Finit from the kernel.  If this file is missing,
+.Nm
+falls back to its built-in default, which can be changed at build time
+using the
+.Cm --with-fstab=/path/to/footab
+configure option.  It is even possible to completely disable the
+built-in using
+.Cm --without-fstab ,
+meaning
+.Nm
+.Sy must
+be started with the
+.Cm finit.fstab
+option.
+.Pp
+Should
+.Nm
+discover that there is no fstab file available, including the fallback,
+it triggers the same failure mode as when
+.Cm fsck
+fails, starting
+.Xr sulogin 8 ,
+if that is availble.  When
+.Cm sulogin
+is missing, or when the user exits from
+.Cm sulogin ,
+the system is rebooted.
+.Pp
+Default fstab, unless changed at build-time:
+.Pa /etc/fstab
 .It Cm finit.status=<on,off>
 Control finit boot progress, including banner.  This used to be called
 .Cm finit.show_status ,
@@ -108,6 +146,10 @@ Before laucnhing services, the system needs to be bootstrapped.  This
 involves mounting all filesystems (not already mounted by an initramfs)
 in
 .Pa /etc/fstab ,
+or any alternate
+.Pa fstab
+file provided on the Finit command line using
+.Cm finit.fstab ,
 ensuring that necessary filesystems like:
 .Pa /dev ,
 .Pa /proc ,