[BUGFIX] Two mailforms on a page cause error on field validation

Check formname for uniqueness and if need append a unique hash.

Change-Id: Ifd33c5130c383f3afc3e5e6d80c1be017f25ec5b
Releases: 6.2, 6.1, 6.0, 4.7, 4.5
Fixes: #16114
Reviewed-on: https://review.typo3.org/12844
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters