syslog: closelog() upon unloading

Not doing this is an error affecting the case library is unloaded
manually.

* Corrected build-error regarding init/fini mechanism for Windows. Mechanism
  will still not work for Windows (someone has to implement it) but change
  will assert not to break build. Note that -DDENABLE_LIBBLADERF_SYSLOG
  option is default OFF and init/fini is so far used only for non-windows
  initialization of logging (i.e. bisectors should be OK).

* Replaced all _linux_ conditionals with corresponding reversed logic
  for Windows, as Windows is really the odd-bird her not Linux. This
  improves transparency.

* Reverting Logging behaviour to before -DDENABLE_LIBBLADERF_SYSLOG was
  introduced to not break code that expect a certain output (bladeRF-CLI
  parsers e.t.a.)