Kernel swap (KSSF): fix context, return code, allocation-failure and redirection handling

[nakatamaho]

Fixes #189.

Kernel-swap (KSSF / CALL /S) fixes on top of current master. None of the touched files changed since com086, so the commits apply cleanly; each commit is self-contained.

1. Environment backup registration (shell/kswap.c): register the segment and size of the environment backup at swap time. The previous code registered an offset (a narrowed far pointer), and a shell
   whose environment was resized with /E: could overrun the early allocation.
2. Dynamic context across reload (shell/kswap.c, include/context.h, tools/kssf.asm): keep the dynamic context (aliases, history, /LOW policy and its counters) allocated across the shell's exit and
   return its ownership to the reloaded shell.
3. Child return code (tools/kssf.asm): preserve the swapped program's DOS return code across the shell reload, so ERRORLEVEL is correct afterwards.
4. Allocation-failure fallback (shell/kswap.c): if the backup allocation fails, execute the command without swapping instead of loading STRINGS while memory is exhausted (which could prompt for a
   resource file); the warning is written through the resident DOS writer.
5. Child status vs. EXEC errors (lib/exec1.c, shell/kswap.c, tools/kssf.asm, criter/context.x, include/misc.h): keep the child's exit status separate from DOS EXEC errors, so a normal exit code
   (for example 7) is no longer reported as an MCB-corruption error.
6. 8086 compatibility (tools/kssf.asm): restrict the expanded reload branch of KSSF to 8086 instructions (CPU 8086), so it runs on 8086/V30-class machines.
7. No swap for redirection, pipes and batch files (shell/command.c): the swap ends the shell process, so DOS closes redirected and piped handles and the batch/FOR context is lost (with /SWAP, prog > file left an empty file). These cases, which docs/k-swap.txt lists as unsupported, now run with an ordinary EXEC.

A documentation commit removes a reference to a test directory that is not part of this tree.

Built with Open Watcom 1.9 (build.sh wc english) and NASM 2.15 (kssf.asm) on current master. Behavior was exercised on an emulated PC-88VA (shell started through KSSF with /SWAP and CALL /S, return to
the reloaded shell, ERRORLEVEL after a swapped program, redirected output). It has not been run on a standard PC with this branch.