Small fixes and cleanups from the failed Win3.1 support (R/W LDT is
completely dropped):
- Never allocate the system descriptors for client (UCODESEL, UDATASEL)
- Set ldt_buffer and pm_stack to NULL after free()
- Zero out the entire client struct on DPMI init, rather than some of
its parts.
- Dsable NULL-ptr fixing code again.


git-svn-id: https://dosemu.svn.sourceforge.net/svnroot/dosemu/trunk@552 7758b857-130e-0410-ab9c-d8735a528d18