Make ReaR disk label configurable (no hardcoded 'REAR-000')

[jsmeix]

Currently the ReaR disk label 'REAR-000'
is hardcoded used at several places in the code.

With a config variable like

REAR_DISK_LABEL="REAR-000"

users could use several differently labeled disks
for example differently labeled USB disks as in
#1520
or a label that they like more in their environment as in
#1492

[gdha]

@jsmeix I would propose to use a name like REAR_USB_DISK_LABEL instead - makes it clear where the name stands for, otherwise, we create more confusion instead of making the end-user easier. Remember, the more choices there are, the number of errors may increase due to misunderstanding the meaning of these variables

[jsmeix]

My initial variable name proposal was just a shot in the dark.

Meanwhile I think this config variable should be named
in compliance with the other config variables that are
used for "rear format" i.e. with a "USB_DEVICE_" prefix.

I would like to avoid "REAR" in ReaR config variable names
because in general it is meaningless to have "REAR" in
ReaR config variable names because obviously all
those variables are about ReaR (and "REAR" is
wrong spelling of "ReaR" ;-)

Accordingly I think we should use in default.conf

USB_DEVICE_FILESYSTEM_LABEL='REAR-000'

It matches well what there is currently described in default.conf

# Filesystem to use for the 'REAR-000' labeled ReaR data partition
...
USB_DEVICE_FILESYSTEM=ext3

And USB_DEVICE_FILESYSTEM_LABEL is technically correct
because that label is set during "rear format" in
format/USB/default/300_format_usb_disk.sh
and in
format/USB/default/350_label_usb_disk.sh
as a label of a particular filesystem (like /dev/sdb2)
and not as a label of a whole disk (like /dev/sdb).

We have already "USB_DEVICE_PARTED_LABEL"
which might be confusing but if things are sufficiently
documented in default.conf everything should be clear.

[gdha]

@jsmeix 👍 I agree.

[jsmeix]

With #1536 merged,
this issue should be fixed.