Fix a couple of uninitialised value warnings from valgrind.

* src/dosext/dpmi/dpmi.c (set_ldt_entry): This wasn't a real problem, I
  think valgrind was just complaining about the padding bits from the
  bitfields not being initialized. I added an initializer to pacify
  valgrind.

* src/plugin/kbd_unicode/serv_xlat.c (keysym_to_keynum): This function
  was not writing the modifiers value when given KEY_VOID. I changed it
  to write MODIFIER_VOID instead in this case.

* src/plugin/kbd_unicode/include/keyboard.h (MODIFIER_VOID): Added for
  keysym_to_keynum. (Perhaps it should have a different value than
  U_VOID, though?)


git-svn-id: https://dosemu.svn.sourceforge.net/svnroot/dosemu/trunk@1894 7758b857-130e-0410-ab9c-d8735a528d18