Debugger: Modernize

Mostly nullptr instead of 0, but also a few bits of collateral damage.

Change-Id: I921991272aca921dcdecf302dfff3716e79dfc24
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/debugger/terminal.cpp

@@ -64,7 +64,7 @@ static QString currentError()
 }
 
 Terminal::Terminal(QObject *parent)
-   : QObject(parent), m_isUsable(false), m_masterFd(-1), m_masterReader(0)
+   : QObject(parent)
 {
 }