Allow absolute paths to prover plugins (#764)

Previously, the `/proverDll` option allowed only relative paths, since
it looked for `/` or `\` _within_ a name, but not at the beginning, to
determine whether the given name was a filename or a portion of an
internal DLL name. This changes it to also interpret a name as a
filename when either of those characters is the first character.

Source/Provers/SMTLib/ProverUtil.cs

@@ -320,7 +320,7 @@ public static ProverFactory Load(string proverName)
       Contract.Ensures(cce.IsNew(Contract.Result<ProverFactory>()) && cce.Owner.New(Contract.Result<ProverFactory>()));
       string /*!*/
         path;
-      if (proverName.IndexOf("/") > 0 || proverName.IndexOf("\\") > 0)
+      if (proverName.IndexOf("/") >= 0 || proverName.IndexOf("\\") >= 0)
       {
         path = proverName;
       }
@@ -367,4 +367,4 @@ public ProverOptionException(string msg)
       Contract.Requires(msg != null);
     }
   }
-}
+}