Change temp file prefixes to be OS-specific hidden files

The previous ".unison." temp file prefix was used on Windows,
where hidden files begin with "~$". This change detects if the
OS is Windows, and will make temp files properly hidden with
a "~$" prefix if so.

Unix file prefixes were updated to begin with ".~", which extends
support for certain backup applications that sync files beginning
with ".", but not ".~".

src/os.ml

@@ -39,7 +39,11 @@ let serverHostName = localCanonicalHostName
 let myCanonicalHostName () =
   if !Trace.runningasserver then serverHostName else Prefs.read clientHostName
 
-let tempFilePrefix = ".unison."
+let tempFilePrefix =
+  match Util.osType with
+  | `Win32 -> "~$unison."
+  | _ -> ".~unison."
+
 let tempFileSuffixFixed = ".unison.tmp"
 let tempFileSuffix = ref tempFileSuffixFixed
 let includeInTempNames s =