Add access modifier support for interface feature

Add ability to change the modifiers for the C# and Java
interface generated when using the %interface macros.

For C# use the 'csinterfacemodifiers' typemap.
For Java use the 'javainterfacemodifiers' typemap.

For example:

  %typemap(csinterfacemodifiers) X "internal interface"

Closes #1874

Doc/Manual/CSharp.html

@@ -240,6 +240,7 @@ javabody                    -> csbody
 javafinalize                -> csfinalize
 javadestruct                -> csdisposing and csdispose
 javadestruct_derived        -> csdisposing_derived and csdispose_derived
+javainterfacemodifiers      -> csinterfacemodifiers
 javainterfacecode           -> csinterfacecode
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