posix/fd: make dup const

It's not strictly semantically correct, but it greatly simplifies reference
parameter binding in win32 memory mapping

posix/fd.cpp

@@ -65,7 +65,7 @@ fd::fd (int _fd):
 
 ///////////////////////////////////////////////////////////////////////////////
 fd
-fd::dup (void)
+fd::dup (void) const
 {
     return dup (m_fd);
 }

posix/fd.hpp

@@ -40,7 +40,7 @@ namespace cruft::posix {
         // possible; one should not be doing this unless it is absolutely
         // required.
         fd (const fd&) = delete;
-        fd dup (void);
+        fd dup (void) const;
         static fd dup (int);
 
         ~fd ();