[BUGFIX] EXT:form - check for compatibility6 if old mailform is used

[BUGFIX] EXT:form - check for compatibility6 if old mailform is used

Check if compatibility6 is loaded if typoscript
"useDefaultContentObject" is set.

Resolves: #69195
Releases: master
Change-Id: Ie1ae530260d6057904a7ed4be2daf573cb358bd0
Reviewed-on: http://review.typo3.org/43472
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Tested-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Michael Oehlhof <typo3@oehlhof.de>
Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>