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

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

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

Update the build files (meson.build and CMakeLists.txt) to use
FSMONITOR_OS_SETTINGS for fsm-settings, matching the approach already
used for fsm-ipc.

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>