backup.pl: Fixes bug13734 - tar treats colon in filename as remote location

backup.pl: Fixes bug13734 - tar treats colon in filename as remote location

- When a user tries to restore on the console from a backup on IPFire that has a colon in
   the filename the tar treats this as meaning that everything after the colon is
   information about a remote location to do the extraction to. This results in a filename
   that cannot be found, and a remote location that is not correct and the tar operation
   fails.
- This has been confirmed by myself.
- If the user tries a restore from a file downloaded to another computer then for most, if
   not all browsers, the colon will have been replaced by an underscore or other character.
   Firefox, Chromium and Vivaldi do this.
- So any backup file that is selected to be restored using the WUI will no longer have a
   colon in the filename.
- This patch adds --force-local to the tar command, which means that tar will treat the
   colon as a character in the filename. This will ensure that if a user has any backup
   files stored on their IPFire system, with a colon in the filename then doing a restore
   from this file will not cause tar to fail.
- The NOW variable is also changed to replace the colon by a dash and to separate the date
   and time by an underscore. This filename will be accepted by browsers, without doing
   any replacements. Tested out with Firefox, Chromium & Vivaldi.
- The above ensures that both the new and old filename versions will work for doing a
   restore.

Fixes: bug13734
Tested-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Reviewed-by: Bernhard Bitsch <bbitsch@ipfire.org>
Tested-by: Bernhard Bitsch <bbitsch@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>