Add process.Release for unix

Signed-off-by: leongross <leon.gross@9elements.com>

src/os/exec.go

@@ -62,6 +62,9 @@ func StartProcess(name string, argv []string, attr *ProcAttr) (*Process, error)
 }
 
 func (p *Process) Wait() (*ProcessState, error) {
+	if p.Pid == -1 {
+		return nil, syscall.EINVAL
+	}
 	return nil, ErrNotImplemented
 }
 
@@ -78,6 +81,13 @@ func Ignore(sig ...Signal) {
 	return
 }
 
+// Release releases any resources associated with the Process p,
+// rendering it unusable in the future.
+// Release only needs to be called if Wait is not.
+func (p *Process) Release() error {
+	return p.release()
+}
+
 // Keep compatibility with golang and always succeed and return new proc with pid on Linux.
 func FindProcess(pid int) (*Process, error) {
 	return findProcess(pid)