[BUGFIX] isValidUrl() idna converts whole URI

GeneralUtility::isValidUrl() idna converts whole URI instead of
domain only.

The expensive idna_convert() is called from isValidUrl(). Instead of
feeding it just the domain part, the whole URI is converted.

When supplying just the domain part, a great speed gain can be seen.

On seriously malformed URLs, parse_url() may return FALSE and emits an
E_WARNING. So we check for that first.

PHP no longer supports the flags FILTER_FLAG_HOST_REQUIRED and
FILTER_FLAG_SCHEME_REQUIRED. A scheme is now required by default. [1]
Return FALSE if the URL does not start with a scheme.

A public GeneralUtility::idnaEncode() method uses a static idna_convert
instance and fetches converted strings from a string cache array
to avoid multiple checks on the same domain.

All manual idna_convert instances are replaced with
GeneralUtility::idnaEncode() calls.

Special characters are not allowed in the URL except in the domain
part [2]. So the test with special characters in the path was removed
from the GeneralUtilityTest class.

[1] http://www.php.net/manual/en/filter.filters.flags.php#107382
[2] http://tools.ietf.org/html/rfc3986#appendix-A

Change-Id: I7a0ab0a399d9d6cf68c824f413be6b6d621947c1
Resolves: #53862
Releases: 6.2, 6.1, 6.0
Reviewed-on: https://review.typo3.org/26501
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers
Reviewed-by: Markus Klein
Tested-by: Markus Klein