Fix the detection of PCI serial ports with sysfs and without udev

Unfortunately, 0973537c5471288726b929250199a41435729bee caused a serious
regression for obtaining detecting PCI serial ports without udev. This was
working before since the hard coded fallback mechanism has been up to this task,
but the sysfs alternative took the precedence over.

A note has also been added to the udev part of the algorithm because we will
need to obtain the vendor and product identifiers in a separate block in the
future. The vid/pid will be fixed in a separate change once that is well-tested.
This patch has no any risk as far as I can tell since it is just adding a new
short block to the existing code without much implication.

I have tested the change on Linux for my use cases with Qt 4 and then 5, and I
cannot find any regression. Everything works as expected both with udev and
without, i.e. with sysfs.

Thanks go to Aaron Linville for reporting the issue.

Task-number: QTBUG-35064
Change-Id: Ieae6f3e94559fb61ede67ff1d8810bb686d46383
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>
Reviewed-by: Aaron Linville <aaron@linville.org>
Obsolete the isValid() method in QSerialPortInfo

Task-number: QTBUG-35184
Change-Id: Ie822943bcaf071024bc4f9782441b0534fcbd43d
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>
Obsolete the QSerialPort::UnknownParity enum value  in QSerialPort

Task-number: QTBUG-35215
Change-Id: I53af65ba485216a6fde238245580fe0422a2ed9a
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>
Deprecate further Unknown* property values similarly to UnknownParity

Task-number: QTBUG-35215
Change-Id: Id8175af00c431cc8f80f05d518b3a2ee0e8ac71b
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>
Do not remove obsolete enum values for gcc and clang

Do not try to work around the gcc/clang limitations with external variables
since some people have some concern that it may break the binary compatibility
due to the underlying representation layout.

Talk to upstream (gcc) if a warning for this is desired. It is out of scope for
us. C++14 will help with the [[deprecated]] attribute, but that is light years
ahead.

Task-number: QTBUG-35354
Change-Id: I1c4e0fb950469adceca85147084fa896503de78b
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Merge remote-tracking branch 'origin/release' into stable

Conflicts:
	src/serialport/qserialport.cpp
	src/serialport/qserialportinfo_unix.cpp

Change-Id: I12658a9f92c91ab83f404c93a311ef947d1f2503