[TASK] CleanUp canonicalizing identifiers in FAL-drivers

In https://review.typo3.org/23398 the possibility was
created, to use case-sensitive fileIdentifiers in
database collations which are not case sensitive. To
do so a new sanitiation of file-identifiers has been
introduced into the drivers.

During the patch-sets a new function has been created
while there would have been already some functions
perfectly fitting that task.

Introducing this new functionality in the old ones
showed, that these are used for non-identifier
path (absolute external), too which created problems.
Therefore this has been split out.

Finally the getAbsolutePath and variants manually
concatenating the basePath have been revised, and
duplicate calls to the canonicalize functions have
been removed.

Releases: 6.2
Resolves: #53604
Change-Id: I415cce898298e9cdaadf7ef4310b15a68c3bb5c6
Reviewed-on: https://review.typo3.org/25382
Reviewed-by: Frans Saris
Tested-by: Frans Saris
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter