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).
  * Dracut have the func emergency_shell() which does a little more.
    For those that don't have it, just run '/bin/sh -i -l'.
  * zfs_set_ifs() needs to be a global func. Use this in zfs_run_cmd()
    before we run the actual command.

Fixing some issues disovered.
  * 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.
  * Create check_boolean() to check if a user config option is
    set ('yes', 'Yes', 'YES' or any combination there of) OR '1'.
    Anything else is considered 'unset'.
  * 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.
  * Move code snippet that fetches and mounts recursive filesystems into
    the new recursive_mount_filesystems() function.
  * 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.
  * 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.
    * Downside: Won't accept pools with spaces in them any more :(.
      Need to find a way to solve this (can't use arrays, because we're
      not running under bash!).
  * Apparently "var > 0" doesn't work. Use 'var -gt 0' instead. But that
    means that 'var' needs to be set.
  * (Re)set IFS in zfs_action() as first step so that command line options
    doesn't get clobbered.
  * Fix checking if pool is imported in import_pool().
  * Remove "sort". Sometimes not available, and not really nessesary.

Signed-off-by: Turbo Fredriksson <[email protected]>
Closes openzfs#3782
Closes pkg-zfs/openzfs#166