fsmonitor-settings: stub in Win32-specific incompatibility checking

fsmonitor-settings: stub in Win32-specific incompatibility checking

Extend generic incompatibility checkout with platform-specific
mechanism.  Stub in Win32 version.

In the existing fsmonitor-settings code we have a way to mark
types of repos as incompatible with fsmonitor (whether via the
hook and IPC APIs).  For example, we do this for bare repos,
since there are no files to watch.

Extend this exclusion mechanism for platform-specific reasons.
This commit just creates the framework and adds a stub for Win32.

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