Support for checking file systems from /etc/fstab

This patch adds support for running fsck on all file systems listed in
/etc/fstab with a passno > 0.  The fsck runs sequentially, starting with
the root fs (passno 1).

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

README.md

@@ -58,7 +58,7 @@ networking, [/etc/finit.d/](doc/config.md#etcfinitd) and the familiar
     # Fallback if /etc/hostname is missing
     host myhostname
     
-    # Devices to fsck at boot
+    # Devices to fsck at boot, only needed if not in /etc/fstab
     check /dev/vda1
     
     # Runlevel to start after bootstrap, runlevel 'S'