Avoid acquiring Giant for some common light-weight or already MPSAFE

Avoid acquiring Giant for some common light-weight or already MPSAFE
fcntl() operations, including:

  F_DUPFD          dup() alias
  F_GETFD          retrieve close-on-exec flag
  F_SETFD          set close-on-exec flag
  F_GETFL          retrieve file descriptor flags

For the remaining fcntl() operations, do acquire Giant, especially
where we call into fo_ioctl() as a result.  We're not yet ready to
push Giant into fo_ioctl().  Once we do, this can all become quite a
bit prettier.