gnupaste: Miscellaneous small fixes

• Fix various minor mistakes in the getopts loop:
  • Add missing $ in front of E_SUCCESS;
  • Suppress error message by getopts and use die() again;
  • Distinguish between wrong (?) and unhandled (*) arguments (add E_INTERNAL for that).
• Remove "long options mandatory arguments" blabla.
• Fix indentation (some lines were using spaces)—if desired, we could also use spaces everywhere. Just don't mix spaces and tabs for indentation 😏.
• Use simple quotes whereever possible.