open gmsh file in binary mode

• otherwise on windows it is opened in text mode
  • ftell/fseek on windows only work correctly on binary files
  • see e.g. https://stackoverflow.com/questions/47256223/why-does-fseek-0-seek-cur-fail-on-windows
  • this makes no difference on posix systems

also: use ftello/fseeko with type off_t instead of ftell/fseek with type long

• https://www.gnu.org/software/libc/manual/html_node/File-Positioning.html
• "using ftello is preferable whenever it is available"