Rewrite unix fork reopen to be compatible with ruby 2.6

On ruby 2.6 the original code would fail specs:

lib/childprocess/unix/fork_exec_process.rb:32:in `reopen': exclusive
access mode is not supported (ArgumentError)

The documentation for reopen shows that it has two ways to call it:

  reopen(other_IO) -> ios
  reopen(path, mode [,opt]) -> ios

With ruby 2.4 and 2.5 calling reopen with a path and no mode seems to
work fine, but with ruby 2.6 this triggers the spec failure.

This commit splits the calls based on stdout/stderr availability so
that both types of reopen calls can get the required parameters. This
fixes the 2.6 specs while being backward compatible with ruby 2.4 and
2.5.

lib/childprocess/unix/fork_exec_process.rb

@@ -29,8 +29,16 @@ def launch_process
           exec_r.close
           set_env
 
-          STDOUT.reopen(stdout || "/dev/null")
-          STDERR.reopen(stderr || "/dev/null")
+          if stdout
+            STDOUT.reopen(stdout)
+          else
+            STDOUT.reopen("/dev/null", "a+")
+          end
+          if stderr
+            STDERR.reopen(stderr)
+          else
+            STDERR.reopen("/dev/null", "a+")
+          end
 
           if duplex?
             STDIN.reopen(reader)