os: add os.open_stdin/0 and os.File.get_line/0

vlib/os/os_test.v

@@ -43,6 +43,23 @@ fn test_open_file() {
 	os.rm(filename)
 }
 
+fn test_file_get_line() {
+	filename := './fgetline.txt'
+	os.write_file(filename, 'line 1\nline 2')
+	mut f := os.open_file(filename, 'r', 0) or {
+		assert false
+		return
+	}
+	line1 := f.get_line() or { '' }
+	line2 := f.get_line() or { '' }
+	f.close()
+	//
+	//eprintln('line1: $line1')
+	//eprintln('line2: $line2')
+	assert line1 == 'line 1\n'
+	assert line2 == 'line 2'
+}
+
 fn test_create_file() {
 	filename := './test1.txt'
 	hello := 'hello world!'