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>