[SECURITY-55]

This patch makes standard post-build action refuse to let you configure a downstream project you cannot currently build.
The one from parameterized-trigger will show an error in the configure screen but still lets you save the configuration; needs an analogous patch to that plugin.
Does not yet protect against POSTing config.xml with the trigger.
(cherry picked from commit 757bc8a5)

Conflicts:

	core/src/main/java/hudson/model/Descriptor.java