Setting process priorities by ksysguard has the flaw that it only
sets priority of a single thread of a multithreaded program. For
example, if I want to put handbrake encoding to the background, it
usually fails as other threads from the same process continue to hog
the CPU. This patch changes the behavior by attempting to change the
priority of all threads (on Linux).

There is a caveat: when the threads' priorities were previously
different, it may mean you change the priority against the logic of
the application. For example some background threads of an
application might end up having the same priority as the
foreground. As a corollary, the call may mean lowering priority of
some threads, but raising it for others and thus fail due to
insufficient capabilities. Still, I think this behavior is actually
the only one usable, until we have individual threads in
ksysguard. Even then, I doubt people will look at the individual
threads when renicing. Till then, the ability to renice a task is a
need which ksysguard should fulfill, but unfortunately doesn't.

Note: this only changes the behavir for remote computers. Local use
needs a similar fix in libksysguard.