[BUGFIX] Skip special doktype parent pages in slug generation

When checking the parent page for an existing slug, the parent page
should not be used if the parent page is a sys folder, spacer or
recycler (pages.doktype), but the traversal goes up.

This makes it a bit easier for editors to work with better-speaking
URLs and keep the rootline traversal as best practice.

Resolves: #86456
Releases: master
Change-Id: I2c46d096e41fb3a325fd42bf86b8968b79d6305b
Reviewed-on: https://review.typo3.org/58473


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>