Apply code review for changes abstract namespace Sockets

At the last stage of developing QTBUG-16090 a few review comments have been postponed to be done in a separate patch.

Tests:
https://codereview.qt-project.org/c/qt/qtbase/+/330032/67/tests/auto/network/socket/qlocalsocket/tst_qlocalsocket.cpp#540

QLocalServer:
https://codereview.qt-project.org/c/qt/qtbase/+/330032/64/src/network/socket/qlocalserver.cpp#110

https://codereview.qt-project.org/c/qt/qtbase/+/330032/64/src/network/socket/qlocalserver_unix.cpp#102
https://codereview.qt-project.org/c/qt/qtbase/+/330032/64/src/network/socket/qlocalserver_unix.cpp#106

QLocalSocket:
https://codereview.qt-project.org/c/qt/qtbase/+/330032/64/src/network/socket/qlocalsocket.cpp#78
https://codereview.qt-project.org/c/qt/qtbase/+/330032/64/src/network/socket/qlocalsocket.cpp#457
https://codereview.qt-project.org/c/qt/qtbase/+/330032/64/src/network/socket/qlocalsocket.cpp#471

This could be done as a subtask (separate patch) :
QLocalSocketPrivate:
https://codereview.qt-project.org/c/qt/qtbase/+/330032/64/src/network/socket/qlocalsocket_p.h#154
I'd go for "an enum return, Error, ParsedAsRegular, ParsedAsAbstract."

This one also :
QLocalServerPrivate:
https://codereview.qt-project.org/c/qt/qtbase/+/330032/57/src/network/socket/qlocalserver_unix.cpp#105
Reuse pathNameForConnection function from
https://codereview.qt-project.org/c/qt/qtbase/+/330032/57/src/network/socket/qlocalsocket_unix.cpp#66