Expose CPU, RAM, username and other system information in QSysInfo

In KDE we have a class called KCPUInfo, which provides information to applications about CPU/OS support for extended instruction sets such as MMX, SSE, 3DNow and Altivec.

Since Qt has optimized paths that make use of these instruction sets, and presumably has its own checks for these features, it would be nice if the results of those checks could be exposed in QSysInfo in a future version of Qt.

Doing so would make it possible for us to remove KCPUInfo in KDE 4, and thus reduce the feature duplication between Qt and kdelibs.

From task 151085:

It would be convenient if QApplication could provide a way to know how much RAM is available in the system, and possibly other memory statistics for the system / current process.

From task 193024:

It would be nice to have information about used/free disk space.

From task 215725:

Per-process information provided by QProcess would be good, ie. a child process could be terminated by the parent if the child is reported to have allocated more than x MB of memory (or other relevant resources).