fs/streams: Move the file streams from the group structure into TLS

This is preparation for flushing streams from user space, like it should
be done.

- Move tg_streamlist (group, kernel space) ->
       ta_streamlist (TLS, user space)
- Access stream list via tg_info in kernel
- Access stream list via TLS in user space
- Remove / rename nxsched_get_streams -> lib_getstreams
- Remove system call for nxsched_get_streams

fs/vfs/fs_fdopen.c

@@ -34,6 +34,7 @@
 #include <nuttx/semaphore.h>
 #include <nuttx/fs/fs.h>
 #include <nuttx/lib/lib.h>
+#include <nuttx/tls.h>
 
 #include "inode/inode.h"
 
@@ -153,11 +154,7 @@ int fs_fdopen(int fd, int oflags, FAR struct tcb_s *tcb,
 
   /* Get the stream list from the TCB */
 
-#ifdef CONFIG_MM_KERNEL_HEAP
-  slist = tcb->group->tg_streamlist;
-#else
-  slist = &tcb->group->tg_streamlist;
-#endif
+  slist = &tcb->group->tg_info->ta_streamlist;
 
   /* Allocate FILE structure */