[PATCH] tgkill patch for safe inter-thread signals

[PATCH] tgkill patch for safe inter-thread signals

This is the updated versions of the patch Ingo sent some time ago to
implement a new tgkill() syscall which specifies the target thread
without any possibility of ambiguity or thread ID wrap races, by passing
in both the thread group _and_ the thread ID as the arguments.

This is really needed since many/most people still run with limited PID
ranges (maybe due to legacy apps breaking) and the PID reuse can cause
problems.