- Fix one more old crasher in the plugin cache loader

- Fix directory separator on Windows
- Some cleanup

io.h on mingw contains a bunch of old functions *for compatibility* which break with the way we deine close at closesocket. So disable all those old stuff.

- Leverage fputc().

If present, assume everything is UTF-8 (obviously);
if not, assume locale charset.

Outputs an error and continue if D-Bus control isn't enabled on the first instance
Removes unneeded NewInstance signal
Closes #505

(and even, it pretty much sucks as we have seen already)
Should fix compilation on FreeBSD+glibc.