Don't close config file when changing preferences

On Linux and MacOS the subsurface_close_conf() doesn't really close the
config file (it flushes writes on MacOS), but on Windows it does
actually close the registry hkey.

Which is bad, if you change the settings multiple times - we assume that
the config file is open the whole time.

So add a "subsurface_flush_conf()" function, and call *that* when
changing configuration parameters.  And call the close function only at
the very end.

Alternatively, maybe we should just open the config file separately
every time. I don't much care, maybe somebody else does.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

linux.c

@@ -43,6 +43,11 @@ const void *subsurface_get_conf(char *name, pref_type_t type)
 	return NULL;
 }
 
+void subsurface_flush_conf(void)
+{
+	/* this is a no-op */
+}
+
 void subsurface_close_conf(void)
 {
 	/* this is a no-op */