Fix basename highlighting for (linux) kernel threads with complex names.

"kworker/0:0H", "migration/2", .., and friends are currently having their basename highlighted in a wrong fashion (highlighter looks for the slash backwards).

IMO the true basename for a kernel thread is what comes before the slash; this slightly pedantic patch takes care of that. Not tested in any platform other than plain GNU/Linux -- I hope I'm not breaking others.