win32/handle: take ownership of any supplied fd

CloseHandle, close, and memory mapping interact poorly so it's a little safer
to dup the fd before taking ownership of it.

io_win32.cpp

@@ -135,10 +135,10 @@ mapped_file::mapped_file (
 
 
 //-----------------------------------------------------------------------------
-mapped_file::mapped_file (const cruft::posix::fd &src,
+mapped_file::mapped_file (cruft::posix::fd const &src,
                           int fflags,
                           int mflags):
-    mapped_file (cruft::win32::handle (reinterpret_cast<HANDLE> (_get_osfhandle (src))),
+    mapped_file (cruft::win32::handle (src.dup ()),
                  fflags,
                  mflags)
 { };

io_win32.hpp

@@ -62,7 +62,7 @@ namespace cruft {
             mapped_file (const std::experimental::filesystem::path &path,
                          int fflags = O_RDONLY,
                          int mflags = PROT_READ);
-            mapped_file (const cruft::posix::fd&,
+            mapped_file (cruft::posix::fd const&,
                          int fflag = O_RDONLY,
                          int mflags = PROT_READ);
 

win32/handle.cpp

@@ -25,6 +25,14 @@ handle::handle (handle &&rhs):
 }
 
 
+//-----------------------------------------------------------------------------
+handle::handle (posix::fd &&rhs):
+    m_native (reinterpret_cast<HANDLE> (_get_osfhandle (rhs.release ())))
+{
+    win32::error::try_value (m_native);
+}
+
+
 //-----------------------------------------------------------------------------
 handle::~handle ()
 {

win32/handle.hpp

@@ -8,12 +8,15 @@
 
 #include <windows.h>
 
+#include "../posix/fd.hpp"
+
 namespace cruft::win32 {
     struct handle {
         handle ();
         explicit handle (HANDLE&&);
         handle (handle&&);
         handle (const handle&) = delete;
+        handle (posix::fd&&);
         ~handle ();
 
         operator HANDLE& (void) &;