fsmonitor--daemon: rename listener thread related variables

fsmonitor--daemon: rename listener thread related variables

Rename platform-specific listener thread related variables
and data types as we prepare to add another backend thread
type.

[] `struct fsmonitor_daemon_backend_data` becomes `struct fsm_listen_data`
[] `state->backend_data` becomes `state->listen_data`
[] `state->error_code` becomes `state->listen_error_code`

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>