os: fix debugger_present() for non Windows OSes

[bstnbuck]

So far the debugger_present() function only works partially. All Unix-based systems use the same procedure. C.ptrace() is used to check whether the process can “debug” itself. If this is the case, no debugger is present, but ptrace is now used as such. If it is called again, the function immediately returns that a debugger is present. However, the appropriate “detach” function cannot be used if the process “follows” itself.

Current behaviour on non Windows systems:

fn main(){
  if os.debugger_present(){
    println("debugger here")
  }else{
    println("no debugger")
  }

  println("###between os.debugger_present()###")

  if os.debugger_present(){
    println("debugger here")
  }else{
    println("no debugger")
  }
}

Output:

no debugger
###between os.debugger_present()###
debugger here   <<< this should also be "no debugger"

A child process must therefore be created to do this. After checking, it is killed again. The resulting return value is checked and returned.

Other procedures are dependent on compatible kernels and are not always functional. Windows has its own APIs and already works without problems.

Relevant literature:

• https://stackoverflow.com/questions/3596781/how-to-detect-if-the-current-process-is-being-run-by-gdb
• https://man7.org/linux/man-pages/man2/ptrace.2.html