backup.pl: Fix #12626 backup/include.user entries will not override backup/exclude

backup.pl: Fix #12626 backup/include.user entries will not override backup/exclude

- Current situation is that any restrictions in the exclude file will not
   be overwritten by the include.user file
- For example the global exclude file has *.tmp preventing any tmp files
   being backed up from the globally included IPFire files
   If a user has some specific tmp files they want to backup and include
   them in the include.user file they will not override the global
   exclude file.
- This fix does the backup of the global and user backups as two separate
   events and then appends them. This means that any tmp files in the
   include.user file will be backed up.
- The backups are created as a global tar file and then have the user
   tar file appended and then the combined file gzipped and given the .ipf
   suffix. This has to be done this was as gzipped files can not be
   appended to each other whereas tar files can.

Fixes: 12626
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>