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

System/Posix/Process.hsc

@@ -29,12 +29,13 @@ module System.Posix.Process (
     -- ** Process environment
     getProcessID,
     getParentProcessID,
-    getProcessGroupID,
 
     -- ** Process groups
-    createProcessGroup,
+    getProcessGroupID,
+    getProcessGroupIDOf,
+    createProcessGroupFor,
     joinProcessGroup,
-    setProcessGroupID,
+    setProcessGroupIDOf,
 
     -- ** Sessions
     createSession,
@@ -58,6 +59,10 @@ module System.Posix.Process (
     getAnyProcessStatus,
     getGroupProcessStatus,
 
+    -- ** Deprecated
+    createProcessGroup,
+    setProcessGroupID,
+
  ) where
 
 #include "HsUnix.h"
@@ -118,11 +123,20 @@ getProcessGroupID = c_getpgrp
 foreign import ccall unsafe "getpgrp"
   c_getpgrp :: IO CPid
 
--- | @'createProcessGroup' pid@ calls @setpgid@ to make
+-- | @'getProcessGroupIDOf' pid@ calls @getpgid@ to obtain the
+--   'ProcessGroupID' for process @pid@.
+getProcessGroupIDOf :: ProcessID -> IO ProcessGroupID
+getProcessGroupIDOf pid =
+  throwErrnoIfMinus1 "getProcessGroupIDOf" (c_getpgid pid)
+
+foreign import ccall unsafe "getpgid"
+  c_getpgid :: CPid -> IO CPid
+
+-- | @'createProcessGroupFor' pid@ calls @setpgid@ to make
 --   process @pid@ a new process group leader.
-createProcessGroup :: ProcessID -> IO ProcessGroupID
-createProcessGroup pid = do
-  throwErrnoIfMinus1_ "createProcessGroup" (c_setpgid pid 0)
+createProcessGroupFor :: ProcessID -> IO ProcessGroupID
+createProcessGroupFor pid = do
+  throwErrnoIfMinus1_ "createProcessGroupFor" (c_setpgid pid 0)
   return pid
 
 -- | @'joinProcessGroup' pgid@ calls @setpgid@ to set the
@@ -131,11 +145,11 @@ joinProcessGroup :: ProcessGroupID -> IO ()
 joinProcessGroup pgid =
   throwErrnoIfMinus1_ "joinProcessGroup" (c_setpgid 0 pgid)
 
--- | @'setProcessGroupID' pid pgid@ calls @setpgid@ to set the
---   'ProcessGroupID' for process @pid@ to @pgid@.
-setProcessGroupID :: ProcessID -> ProcessGroupID -> IO ()
-setProcessGroupID pid pgid =
-  throwErrnoIfMinus1_ "setProcessGroupID" (c_setpgid pid pgid)
+-- | @'setProcessGroupIDOf' pid pgid@ calls @setpgid@ to set the
+--   'ProcessGroupIDOf' for process @pid@ to @pgid@.
+setProcessGroupIDOf :: ProcessID -> ProcessGroupID -> IO ()
+setProcessGroupIDOf pid pgid =
+  throwErrnoIfMinus1_ "setProcessGroupIDOf" (c_setpgid pid pgid)
 
 foreign import ccall unsafe "setpgid"
   c_setpgid :: CPid -> CPid -> IO CInt
@@ -396,3 +410,27 @@ foreign import ccall unsafe "exit"
   c_exit :: CInt -> IO ()
 
 -- -----------------------------------------------------------------------------
+-- Deprecated or subject to change
+
+{-# DEPRECATED createProcessGroup "This function is subject to change in future versions." #-}
+-- | @'createProcessGroup' pid@ calls @setpgid@ to make
+--   process @pid@ a new process group leader.
+--   This function is currently deprecated,
+--   and might be changed to making the current
+--   process a new process group leader in future versions.
+createProcessGroup :: ProcessID -> IO ProcessGroupID
+createProcessGroup pid = do
+  throwErrnoIfMinus1_ "createProcessGroup" (c_setpgid pid 0)
+  return pid
+
+{-# DEPRECATED setProcessGroupID "This function is subject to change in future versions." #-}
+-- | @'setProcessGroupID' pid pgid@ calls @setpgid@ to set the
+--   'ProcessGroupID' for process @pid@ to @pgid@.
+--   This function is currently deprecated,
+--   and might be changed to setting the 'ProcessGroupID'
+--   for the current process in future versions.
+setProcessGroupID :: ProcessID -> ProcessGroupID -> IO ()
+setProcessGroupID pid pgid =
+  throwErrnoIfMinus1_ "setProcessGroupID" (c_setpgid pid pgid)
+
+-- -----------------------------------------------------------------------------