Refactoring of common code between the SYSV init and initramfs scripts.

* Move code from the SYSV init and initramfs scripts that is better served
  in a common code base library (zfs-functions).
* Use "${VAR}" instead of "$VAR".
* Add some comments to functions that didn't have any.
* Create zfs_run_cmd() which records stderr, exit code etc.
* zfs_set_ifs() needs to be a global func. Use this in zfs_run_cmd()
  before we run the actual command.
* Dracut have the func emergency_shell() which does a little more.
  For those that don't have it, just run '/bin/sh -i -l'.
* Minor fix/change to read_mtab():
  * Strip control characters (space - \040) from /proc/mounts GLOBALY,
    not just first occurrence.
  * Don't replace unprintable characters ([/-. ]) for use in the variable
    name with underscore. No need, just remove them all together.
* Move code snippet that fetches and mounts recursive filesystems into
  the new recursive_mount_filesystems() function.
* Instead of using ZFS_CMD+zfs_action()+zfs_log_failure_msg() etc
  each and every time, use the more generic zfs_action() that does
  most of this for us.
* Put the check for zfs debugging in check_zfs_debug(). Use this in
  zfs_action() to check if we should output the message we're called
  with.