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

include/sys/syscall_lookup.h

@@ -268,7 +268,6 @@ SYSCALL_LOOKUP(futimens,                   2)
 
 #ifdef CONFIG_FILE_STREAM
   SYSCALL_LOOKUP(fs_fdopen,                4)
-  SYSCALL_LOOKUP(nxsched_get_streams,      0)
 #endif
 
 #ifndef CONFIG_DISABLE_MOUNTPOINT