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>

doc/config.md

@@ -27,7 +27,8 @@ like what a SysV init start script usually does.
 Syntax:
 
 * `check <DEV>`  
-  Run fsck on a file system before mounting it
+  Run fsck on a file system.  Only needed if not listed in `/etc/fstab`,
+  Finit 3.0 and later check all file systems automatically at boot.
 
 * `module <MODULE>`  
   Load a kernel module, with optional arguments