Do not exclude '/var/tmp/rear.*' in default.conf (#3229)

Revert the actual change in
#3224
and have BACKUP_PROG_EXCLUDE in default.conf
same as it was before because the current BUILD_DIR
gets automatically excluded in usr/sbin/rear via
BACKUP_PROG_EXCLUDE+=( "$BUILD_DIR" )
and it is not ReaR's task to exclude possibly lefover
older ReaR working areas because in general
files in /var/tmp are persistent which need to be
cleaned up deliberately by the user
(and not by ReaR from behind via its backup).
Described things more clearly in the comment.

usr/share/rear/conf/default.conf

@@ -1525,23 +1525,27 @@ BACKUP_PROG_DECRYPT_OPTIONS="/usr/bin/openssl des3 -d -k "
 # for "rear recover" so that the backup can be found and restored
 # (i.e. during "rear recover" a dynamic name does not make sense).
 BACKUP_PROG_ARCHIVE="${BACKUP_PROG_ARCHIVE:-backup}"
-# BACKUP_PROG_EXCLUDE is an array of strings that get written into a backup-exclude.txt file
-# that is used e.g. in 'tar -X backup-exclude.txt' to get things excluded from the backup.
+# BACKUP_PROG_EXCLUDE is an array of strings that get written into a backup-exclude.txt file,
+# which is used e.g. in 'tar -X backup-exclude.txt' to get things excluded from the backup.
 # Quoting of the BACKUP_PROG_EXCLUDE array members avoids bash pathname expansion
-# when bash pathname expansion is not wanted for the BACKUP_PROG_EXCLUDE array members.
+# so e.g. 'tar' gets the unexpanded BACKUP_PROG_EXCLUDE array elements.
 # Via the '/directory/*' form the plain directory is still included in the backup
 # which is mandatory when directories have special owner,group,permissions set
 # to ensure directories get restored from the backup with right owner,group,permissions.
-# By default /var/tmp/rear.* is excluded because ReaR uses /var/tmp/rear.* as TMPDIR
-# (see TMPDIR and KEEP_BUILD_DIR above) to avoid that one gets ReaR's whole BUILD_DIR
-# at least of the current "rear mkbackup" run included in the backup.
-# Also ReaR's VAR_DIR/output is excluded because in particular with OUTPUT=ISO
-# the ReaR recovery system ISO image var/lib/rear/output/rear-HOSTNAME.iso
-# could be rather big and is not needed in the backup.
+# ReaR's VAR_DIR/output is excluded to avoid ISOs being backed up with OUTPUT=ISO
+# because the ISO image var/lib/rear/output/rear-HOSTNAME.iso could be rather big
+# especially since the backup can be embedded inside (cf. BACKUP_URL=iso:///backup)
+# and the ISO image is generally kept and used outside the backup (cf. OUTPUT_URL).
+# ReaR's current working/build area (see TMPDIR and KEEP_BUILD_DIR above)
+# which is normally /var/tmp/rear.XXXXXXXXXXXXXXX (by default TMPDIR is /var/tmp)
+# is automatically excluded in /usr/sbin/rear via BACKUP_PROG_EXCLUDE+=( "$BUILD_DIR" )
+# but the whole /var/tmp should not be excluded from the backup by default here
+# because /var/tmp is for persistent temporary files (see "man file-hierarchy")
+# so in general files in /var/tmp need to be cleaned up deliberately by the user.
 # In /etc/rear/local.conf use BACKUP_PROG_EXCLUDE+=( '/this/*' '/that/*' )
-# to specify your particular items that should be excluded from the backup in addition to what
-# gets excluded from the backup by default here (see also BACKUP_ONLY_EXCLUDE below):
-BACKUP_PROG_EXCLUDE=( '/tmp/*' '/dev/shm/*' '/var/tmp/rear.*' "$VAR_DIR/output/*" )
+# to specify your particular items that should be excluded from the backup in addition
+# to what gets excluded by default here (see also BACKUP_ONLY_EXCLUDE below):
+BACKUP_PROG_EXCLUDE=( '/tmp/*' '/dev/shm/*' "$VAR_DIR/output/*" )
 # BACKUP_PROG_INCLUDE is an array of strings that get written into a backup-include.txt file
 # that is used e.g. in 'tar -c $(cat backup-include.txt)' to get things included in the backup.
 # Quoting of the BACKUP_PROG_INCLUDE array members avoids bash pathname expansion