process.PidExists() is inefficient, should only check given PID

[Lomanic]

Instead of listing every processes and iterating over each of them it should use the OS capabilities to only check the given pid:

• Linux: check if /proc/$PID exists
• BSDs/darwin: check the result of ps -o pid= -p $PID
• Windows: OpenProcess + GetExitCodeProcess
• or Linux+BSDs+Darwin: try kill -s 0 $PID, and check the error returned

This might let us systematically check process existence at the beginning of every process function like Python psutil does/when an error is encountered, for better error messages. As discussed in #605 (comment)

Note: depending of the implementation, it might still be more efficient to keep the current "get all running processes and iterate over them" approach to check for PID existence, this should always be benchmarked.

[Lomanic]

Looks like as simple as using os.FindProcess and process.Signal(syscall.Signal(0)), only problem is on Windows os.FindProcess doesn't do as many checks as Python psutil, so we will still need to redefine the same function in process/process_posix.go and process/process_windows.go (using syscalls there).