This patch adds gdb_fildes_t support to gdbserver for later use by the
gdbserver multi-client patch.  That patch enables multiple clients
with each client associated with its corresponding file desc.   This
patch adds the corresponding file desc to the client_state and adds a
file desc parameter to the gdbserver I/O ops.  It also adds a minimal
multi_client_states which currently just manages a single client_state
but later will be expanded to be a collection of multiple
client_states where each client_state is the state for an individual
client.  In contrast to the previous patch, this patch
  1) uses client_state.remote_desc in remote-utils.c instead of the 
static version
  2) no longer uses the function get_client_state but instead defines 
get_client_state
     using multi_client_states
  The gdbserver multi-client patch is mentioned in:
  https://sourceware.org/ml/gdb-patches/2019-08/msg00577.html


> Will remote_desc be going away?  And hopefully get_remote_desc is also
> just a stepping stone?

remote_desc is now gone as mentioned in 1 above


> Can get_client_state be made static now?
> If not, why not?

get_client_state is now redefined as mentioned in 2) above

Thanks Tom!