Add "x" mode option for IO.open

From CRuby 2.6 feature.

This is the "x" mode (`O_EXCL`; exclusive create) of `fopen(3)` introduced in C11.

ref. https://bugs.ruby-lang.org/issues/11258

mrbgems/mruby-io/src/io.c

@@ -149,6 +149,10 @@ io_modestr_to_flags(mrb_state *mrb, const char *mode)
         flags |= O_BINARY;
 #endif
         break;
+      case 'x':
+        if (mode[0] != 'w') goto modeerr;
+        flags |= O_EXCL;
+        break;
       case '+':
         flags = (flags & ~OPEN_ACCESS_MODE_FLAGS) | O_RDWR;
         break;

mrbgems/mruby-io/test/file.rb

@@ -260,4 +260,30 @@ assert('File.chmod') do
   end
 end
 
+assert('File.open with "x" mode') do
+  File.unlink $mrbtest_io_wfname rescue nil
+  assert_nothing_raised do
+    File.open($mrbtest_io_wfname, "wx") {}
+  end
+  assert_raise(RuntimeError) do
+    File.open($mrbtest_io_wfname, "wx") {}
+  end
+
+  File.unlink $mrbtest_io_wfname rescue nil
+  assert_nothing_raised do
+    File.open($mrbtest_io_wfname, "w+x") {}
+  end
+  assert_raise(RuntimeError) do
+    File.open($mrbtest_io_wfname, "w+x") {}
+  end
+
+  assert_raise(ArgumentError) do
+    File.open($mrbtest_io_wfname, "rx") {}
+  end
+
+  assert_raise(ArgumentError) do
+    File.open($mrbtest_io_wfname, "ax") {}
+  end
+end
+
 MRubyIOTestUtil.io_test_cleanup