Merge branch 'grubba/fdlib' into 8.1
I/O [NT]: Moved FD_SETSIZE definition to global.h.
I/O [NT]: Use new low-level functions from Process.
I/O [NT]: Added some assertions.
I/O [NT]: Added serialization of access to fd handles.
I/O [NT]: Use the new low-level functions.
I/O [NT]: Added some low-level fd handling functions.
Build [NT]: Fix typo of EACCES.
I/O [NT]: Document some internal data structures.