TerminalControllingProcess: don't use qWarning in the child process

qWarning() and all of QMessageLogger will get to qFormatLogMessage(),
which locks a mutex. Additionally, qWarning may call a number of
different backends that, in turn, may have mutexes of their own. Locking
mutexes in child processes between fork() and execve() is a big no-no:
it may have been locked by another thread before fork(), so it's still
locked in the child process and will never get unlocked. Result:
deadlock.

Plus, qWarning reacts to QT_FATAL_WARNINGS, which I guess was not
intended for this class.

So just use a plain perror(), which is guaranteed by POSIX to be
"MT-Safe race:stderr".

Change-Id: I4e559af2a9a1455ab770fffd15f4a37a3fd113ca
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/libs/utils/qtcprocess.cpp

@@ -39,6 +39,7 @@
 #include <qt_windows.h>
 #else
 #include <errno.h>
+#include <stdio.h>
 #include <unistd.h>
 #endif
 
@@ -1226,7 +1227,7 @@ void QtcProcess::setupChildProcess()
     if (m_lowPriority) {
         errno = 0;
         if (::nice(5) == -1 && errno != 0)
-            qWarning("Failed to set nice value. Error: %d", errno);
+            perror("Failed to set nice value");
     }
 #endif
     QProcess::setupChildProcess();