Fix dozens of memory violations in builtins, mostly detected by the
smalloc memory allocator. The fixes include (but not limited to the)
following:
- Cease the usage of the low mem considerably by calling the helpers
directly.
- Protect do_call_back() against re-entering. This was happening
when eg. cmdline is interrupted by ^C, it was not able to recover.
- Track all the allocations of load_and_run_DOS_program() to free it
on termination (leaks).
- On dos_read()/dos_write() set int0x23 to our return point, so that
dosemu can recover even in case of ^C.
- Fix com_strfree() to free the region of a proper size.
- dos_helper_r is no longer necessary.
- com_dosread() was not checking the size specified by caller -
memory corruption.
- memory corruption with builtin_name, no longer use DOS mem for it.
- etc...


git-svn-id: https://dosemu.svn.sourceforge.net/svnroot/dosemu/trunk@669 7758b857-130e-0410-ab9c-d8735a528d18