Support for btrfs snapshotsSnapshots

[DBa20-24]

How this change works:

• the "snapshot_pre" extension:
• • sets "SNAPDEST" variable to /.snapshot-raspiBackup-$BACKUPFILE
• • stops the services (getting the list of commands from the STOPSERVICES variable, this the change to export it!),
• • creates a snapshot (in $SNAPDEST)
• • starts the services again (getting commands from STARTSERVICES variable)
• • and changes "STARTSERVICES" variable to snapshot deletion and empties the "STOPSESRVICES" variable to prevent the services from being stopped again
• (it should also check the USE_FS_SNAPSHOTS command to skip above actions if it is not set - this is for the next change)

The main script:

• accepts "--snapshot/--snapshots" to activate the new functionality (sets USE_FS_SNAPSHOTS)
• when mounting a filesystem for backup, checks the FS type
• if a btrfs is encountered and USE_FS_SNAPSHOTS activated and SNAPDEST is set (i.e. extension is loaded), an additional mount option is passed to the mount command: "subvol=$SNAPDEST", which mounts the snapshot rather than the live filesystem
• everythign else pretty much unchanged

[framps]

I had various comments. Please check 😉

[framps]

You added raspiBackup.sh.swap in your PR. Please remove this file.

[DBa20-24]

Swap file removed

[DBa20-24]

There was a typo, corrected now