[BUGFIX] Allow URL path segments like "typo3"

Tighten the RewriteRule for static resource passthrough to allow for
speaking URL path segments like "typo3", "fileadmin", etc. instead of
causing a 404 error.

This issue was introduced with https://review.typo3.org/39254/

Resolves: #76928
Releases: master, 7.6, 6.2
Change-Id: I9815b7626d6a33677a3f971f452c0600141632b4
Reviewed-on: https://review.typo3.org/48788


Tested-by: Bamboo TYPO3com <info@typo3.com>
Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>