Make independent implementation of QSerialPortInfo for OS X

Building in OS X uses some shared code from the serialportinfo_unix.cpp
module with the OS X specific code from the serialportinfo_mac.cpp module.

Thus, all code of the serialportinfo_unix.cpp module, which not related
with the OS X, is shielded by a macro. It adds an excessive garbage for
readability in this module and also some confusion in the *.pri file.

It makes sense to make implementation of the serialportinfo_mac.cpp
module completely independent, that will simplify maintaining of source
code.

Besides are added tests which can reveal declared but not implemented
methods in building, and also to check a correctness of default values
at running.

Tested build on OS X 10.8.4 with Qt4, an then on Android with Qt5.

Change-Id: I67935b64e2b623fb8d4c14d59e1b87f1eac71c3e
Reviewed-by: Bernard Pratz <guyzmo+qt@m0g.net>
Reviewed-by: Peter Kümmel <syntheticpp@gmx.net>
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>