Correct out-of order lock acquisition.

Previously we had:

TFile::Open
 TFile::GetType
  TUnixSystem::IsPathLocal
   TSystem::FindHelper
    TPluginHandler::CheckForExecPlugin
     ** take the TPluginHandler lock **
      TPluginHandler::SetupCallEnv
       TClass::GetClass
        ** take the ROOT/Interpreter lock **
vs

TFile::Open
 TPluginHandler::ExecPluginImpl
  ** take the ROOT/Interpreter lock **
   TMethodCall::Execute
    ... interpreter layer
    TPluginManager::AddHandler
     ** take the TPluginHandler lock **

core/base/src/TPluginManager.cxx

@@ -280,6 +280,11 @@ Bool_t TPluginHandler::CheckForExecPlugin(Int_t nargs)
 
    if (fCanCall == 0) {
       // Not initialized yet.
+      // SetupCallEnv is likely to require/take the interpreter lock.
+      // Grab it now to avoid dead-lock.  In particular TPluginHandler::ExecPluginImpl
+      // takes the gInterpreterMutex and *then* call (indirectly) code that
+      // take the gPluginManagerMutex.
+      R__LOCKGUARD2(gInterpreterMutex);
       R__LOCKGUARD2(gPluginManagerMutex);
 
       // Now check if another thread did not already do the work.