Adds an intermediate stage in closing an fd.

Adds a new bit to mark an fd being in shutdown. This is a state
during close that marks the fd as being bad, with all kernel calls
(from other threads) returning EBADF if they refer to this fd.
At the same time the fd is not marked as free for reuse by another
open.

This fixes the race condition in
#440

src/freertos_drivers/common/Device.cxx

@@ -144,6 +144,7 @@ int Device::close(struct _reent *reent, int fd)
         // stdin, stdout, and stderr never get closed
         return 0;
     }
+    files[fd].inshdn = true;
     int result = f->dev->close(f);
     if (result < 0)
     {

src/freertos_drivers/common/Devtab.hxx

@@ -62,6 +62,7 @@ struct File
     off_t offset; /**< current offset within file */
     int flags;    /**< open flags */
     uint8_t inuse  : 1; /**< true if this is an open fd. */
+    uint8_t inshdn : 1; /**< true if this fd is in shutdown. */
     uint8_t device : 1; /**< true if this is a device, false if file system */
     uint8_t dir    : 1; /**< true if this is a directory, else false */
     uint8_t dirty  : 1; /**< true if this file is dirty and needs flush */

src/freertos_drivers/common/Fileio.cxx

@@ -51,6 +51,7 @@ int FileIO::fd_alloc(void)
         if (files[i].inuse == false)
         {
             files[i].inuse = true;
+            files[i].inshdn = false;
             files[i].device = true;
             files[i].dir = false;
             files[i].dirty = false;
@@ -85,7 +86,7 @@ File* FileIO::file_lookup(int fd)
         errno = EBADF;
         return nullptr;
     }
-    if (files[fd].inuse == 0)
+    if (files[fd].inuse == 0 || files[fd].inshdn == 1)
     {
         errno = EBADF;
         return nullptr;