Change the POSIX process group API. (trac #5167)

Make it possible to query the process group of an existing
process (through 'getProcessGroupIDOf') and try to make
function names more consistent. Here is the full list of
API changes in System.Posix.Process in this patch:

getProcessGroupID     => same
getProcessGroupIDOf   => new
createProcessGroup    => deprecated
createProcessGroupFor => new
joinProcessGroup      => same
setProcessGroupID     => deprecated
setProcessGroupIDOf   => new