[TYPO3-core] Fix #11669: Provide a reason for why $TSFE->set_no_cache() has been triggered

[Michael Stucki]

This is a SVN patch request.

Type: Feature

Branches: Trunk

Bugtracker reference:
http://bugs.typo3.org/view.php?id=11669

Problem:
$TSFE->set_no_cache() uses t3lib_div::sysLog() to log why the caching
had been disabled.

Currently this is always done by identifying the trigger using a
backtrace. This is not always helpful, in fact a more precise message
would be much more helpful.

Solution:
Introduce a new parameter $reason. It stays optional, so if it's not
supplied, the old behaviour remains (identify and output the trigger).

Greetings, Michael
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