[BUGFIX] Namespace forgotten t3lib_formmail

Class t3lib_formmail was deprecated with 6.0 and not namespaced,
but it is still used by the default mail content object and
handles submitted data.
The patch post-namespaces the class for now and removes the
deprecation.

Fixes: #47157
Releases: 6.1, 6.0

Change-Id: I904020fbf269ea77d1cd45bd02cd7ce459e92eb0
Reviewed-on: https://review.typo3.org/19828
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
[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/19831
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn