DEBUG trap: restore status 2 trigger to skip command (re: d00b4b39)

So now we know what that faulty check for shp->indebug in sh_trap()
was meant to do: it was meant to pass down the trap handler's exit
status, via sh_debug(), down to sh_exec() (xec.c) so that it could
then skip the execution of the next command if the trap's exit
status is 2, as documented in the manual page. As of d00b4b39, exit
status 2 was not passed down, so this stopped working.

This commit reinstates that functionality, but without the exit
status bug in command substitutions caused by the old way.

src/cmd/ksh93/sh/fault.c: sh_trap():
- Save the trap's exit status before restoring the parent
  envionment's exit status. Make this saved exit status the return
  value of the function. (This does not break anything, AFAICT; the
  majority of sh_trap() calls ignore the return value, and the few
  that don't ignore it seem to expect it to return exactly this.)

src/cmd/ksh93/sh/xec.c: sh_exec():
- The sh_trap() fix has one side effect: whereas the exit status of
  a skipped command was always 2 (as per the trap handler), now it
  is always 0, because it gets reset in sh_exec() but no command is
  executed. That is probably not a desirable change in behaviour,
  so let's fix that here instead: set sh.exitval to 2 when skipping
  commands.

src/cmd/ksh93/sh.1:
- Document that ${.sh.command} shell-quotes its arguments for use
  by 'eval' and such. This fact was not documented anywhere, AFAIK.

src/cmd/ksh93/shell.3:
- Document that $? (exit status) is made local to trap handlers.
- Document that sh_trap() returns the trap handler's exit status.

src/cmd/ksh93/tests/basic.sh:
- Add test for this bug.
- Add a missing test for the exit status 255 functionality (if a
  DEBUG trap handler yields this exit status and we're executing a
  function or dot script, a return is triggered).

Fixes: https://github.com/ksh93/ksh/issues/187

src/cmd/ksh93/sh.1

@@ -1561,6 +1561,8 @@ When processing a
 .B DEBUG
 trap, this variable contains the current command line
 that is about to run.
+Each argument is shell-quoted as necessary
+so that the value is safe for being evaluated by the shell.
 .TP
 .B .sh.edchar
 This variable contains the value of the keyboard character
@@ -7429,7 +7431,7 @@ then
 will be executed before each command.
 The variable
 .B .sh.command
-will contain the contents of the current command line
+will contain the shell-quoted arguments of the current command line
 when
 .I action\^
 is running.