Fix field splitting bug triggered by DEBUG trap

An unquoted variable expansion evaluated in a DEBUG trap action
caused IFS field splitting to be deactivated in code executed after
the trap action. Thanks to Koichi Nakashima for the reproducer:

| v=''
| trap ': $v' DEBUG
| A="a b c"
| set -- $A
| printf '%s\n' "$@"
|
| Expected
|
| a
| b
| c
|
| Actual
|
| a b c

src/cmd/ksh93/sh/fault.c: sh_trap():
- Remove incorrect save/restore of sh.ifstable, the internal state
  table for field splitting. This reverts three lines added in ksh
  93t+ 2009-11-30. Analysis: As an expansion is split into fields
  (macro.c, lines 2367-2471), sh.ifstable is modified. If that
  happens within a DEBUG trap, any modifications in ifstable are
  undone by the restoring memccpy, leaving an inconsistent state.

src/cmd/ksh93/COMPATIBILITY:
- Document the DEBUG trap fixes, particularly the incorrect
  inheritance by subshells and functions that some scripts may now
  rely on because this bug is so longstanding. (re: 2a835a2d)

src/cmd/ksh93/tests/basic.sh:
- Add relevant tests.

Resolves: https://github.com/ksh93/ksh/issues/155

TODO: add a -T (-o functrace) option as in bash, which should allow
subshells and ksh-style functions to inherit DEBUG traps.

P.S.: The very handy multishell repo allows us to use 'git blame'
to trace the origin of the recently fixed DEBUG trap bugs.

The off-by-one error causing various bugs, reverted in 2a835a2d,
was introduced in ksh 93t 2008-07-25:
8e947ccf
(fault.c, line 321)

The incorrect check causing the exit status bug, reverted in
d00b4b39, was introduced in ksh 93t 2008-11-04:
b1ade268
(fault.c, line 459)

The ifstable save/restore causing the field splitting bug, reverted
in this commit, was introduced in ksh 93t+ 2009-11-30:
53d9f009
(fault.c, lines 440, 444, 482)

So all the bugs reported in #155 were fixed by simply reverting
these specific changes. I think that they are some experiments that
the developers simply forgot to remove. I've suspected such a thing
multiple times before. ksh93 was developed by researchers who were
genius innovators, but incredibly sloppy maintainers.

NEWS

@@ -3,11 +3,19 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-01-24:
+
+- Fixed: an unquoted variable expansion evaluated in a DEBUG trap action caused
+  IFS field splitting to be deactivated in code executed after the trap action.
+  This bug was introduced in ksh 93t+ 2009-11-30.
+
 2021-01-23:
 
 - Fixed: when the DEBUG trap was redefined in a subshell, the DEBUG trap in
   the parent environment was corrupted or the shell crashed.
   When a redirection was used in a DEBUG trap action, the trap was disabled.
+  DEBUG traps were also incorrectly inherited by subshells and ksh functions.
+  All this was caused by a bug introduced in ksh 93t 2008-07-25.
 
 2021-01-22:
 
@@ -18,6 +26,7 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 
 - Fixed: executing a DEBUG trap in a command substitution had side effects
   on the exit status ($?) of non-trap commands.
+  This bug was introduced in ksh 93t 2008-11-04.
 
 - The typeset builtin command now gives an informative error message if an
   incompatible combination of options is given.