[BUGFIX][FAL] isValidFilename() fails with broken PCRE

Some php versions like debian squeeze 5.3.3 do not have proper unicode
support for PCRE enabled. In effect, character classes like [:alnum:]
do not contain unicode characters. Therefore, test
filenamesAreCorrectlyValidated with data set "filename with german
umlauts" fails on those systems.

The patch changes the according regex in t3lib_file_Driver_AbstractDriver
to other, fully supported character classes.

Change-Id: I49c9c2488536577ba77c5065e96a5ef4859e03da
Fixes: #39123
Releases: 6.0
Reviewed-on: http://review.typo3.org/12924
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Daniel Lorenz
Tested-by: Daniel Lorenz
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn