[BUGFIX] Deprecate forgotten t3lib classes

Nearly all classes in t3lib were namespaced with 6.0 except some
old deprecated functionality. Some of the files miss a proper
deprecation. This is fixed with the patch:

* Deprecate t3lib_readmail, t3lib_syntaxhl, t3lib_xml
* Move some forgotten constants from t3lib_svbase to bootstrap

Change-Id: I15a7c99e4ca254e22aa4037aed0e76e9495ef0e3
Fixes: #46278
Releases: 6.1, 6.0
Reviewed-on: https://review.typo3.org/18905
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn