fsmonitor: rename fsm-ipc-darwin.c to fsm-ipc-unix.c

fsmonitor: rename fsm-ipc-darwin.c to fsm-ipc-unix.c

The fsmonitor IPC path logic in fsm-ipc-darwin.c is not
Darwin-specific and will be reused by the upcoming Linux
implementation.  Rename it to fsm-ipc-unix.c to reflect that it
is shared by all Unix platforms.

Introduce FSMONITOR_OS_SETTINGS (set to "unix" for non-Windows, "win32"
for Windows) as a separate variable from FSMONITOR_DAEMON_BACKEND so
that the build files can distinguish between platform-specific files
(listen, health, path-utils) and shared Unix files (ipc, settings).

Move fsm-ipc to the FSMONITOR_OS_SETTINGS section in the Makefile, and
switch fsm-path-utils to use FSMONITOR_DAEMON_BACKEND since path-utils
is platform-specific (there will be separate darwin and linux versions).

Based-on-patch-by: Eric DeCosta <edecosta@mathworks.com>
Based-on-patch-by: Marziyeh Esipreh <marziyeh.esipreh@gmail.com>
Signed-off-by: Paul Tarjan <github@paulisageek.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>