Improves systemd.path unit

[Antynea]

• grub-btrfs.path : improves unit
  Allows the unit to start and stop automatically when the mount point is detected.
  If the mount point is already mounted when the unit is activated systemctl enable grub-btrfs.path,
  it will be necessary to start the unit for monitoring to begin systemctl start grub-btrfs.path.

  • DefaultDependencies=no
    Prevents systemd from automatically generating Wants= or Requires= or After=.
  • Requires=\x2esnapshots.mount
    Ensures unit is started if mount point exists.
  • After=\x2esnapshots.mount
    Unit start after the mounting point exists.
  • BindsTo=\x2esnapshots.mount
    If the mount point is removed, the unit will stop.
  • PathModified=/.snapshots
    The monitored folder containing the snapshots.
  • WantedBy=\x2esnapshots.mount
    If the mount point exists, unit start automatically. (only if unit is activated before the mount point is mounted)

• Readme : Update "Automatically update grub" section
  Update "Automatically update grub" section following the recent change in the grub-btrfs.path unit.

Fix: #138

[Antynea]

New commit:

• Readme: Add a new paragraph in Automatically update grub
  • Add information to be able to update the grub-btrfs menu every time the system is restart/shutdown.

This feature isn't included in the grub-btrfs package, because i think, systemd doesn't allow us to fix a malfunction if the command fails during a system restart/shutdown.
Or, I don't know how to do it. (don't blame systemd)