v6.26 (#3020)

+ DietPi-Software | XRDP: On Stretch, install version from backports, which resolves missing /etc/profile (e.g. $PATH) load and failing install+service if IPv6 is disabled: #3017
+ DietPi-Pre-patch | XRDP: On Stretch allow upgrade of XRDP from backports to resolve certain issues
+ DietPi-Pre-patch | Remove pre-patch 7 which will be applied from pre-patch_file of jessie-support branch
+ DietPi-Pre-patch | Simplify pre-patch 6: Every Jessie system which reaches this pre-patch_file is obviously not yet on jessie-support branch
+ DietPi-Pre-patch | Minor coding
+ CHANGELOG | Complete spell check and correction