Fix incorrect exec optimisation with monitor/pipefail on

Reproducer script:
    tempfile=/tmp/out2.$$.$RANDOM
    bintrue=$(whence -p true)
    for opt in monitor pipefail
    do
            (
                    set +x -o "$opt"
                    (
                            sleep .05
                            echo "ok $opt" >&2
                    ) 2>$tempfile | "$bintrue"
            ) &
            wait
            cat "$tempfile"
            rm -f "$tempfile"
    done

Expected output:
    ok monitor
    ok pipefail

Actual output:
    (none)

The 'monitor' and 'pipefail' options are supposed to make the shell
wait for the all commands in the pipeline to terminate and not only
the last component, regardless of whether the pipe between the
component commands is still open. In the failing reproducer, the
dummy external true command is subject to an exec optimization, so
it replaces the subshell instead of forking a new process. This is
incorrect, as the shell is no longer around to wait for the
left-hand part of the pipeline, so it continues in the background
without being waited for. Since it writes to standard error after
.05 seconds (after the pipe is closed), the 'cat' command reliably
finds the temp file empty. Without the sleep this would be a race
condition with unpredictable results.

Interestingly, this bug is only triggered for a (background
subshell)& and not for a forked (regular subshell). Which means the
exec optimization is not done for a forked regular subshell, though
there is no reason not to. That will be fixed in the next commit.

src/cmd/ksh93/sh/xec.c: sh_exec():
- case TFORK: Never allow an exec optimization if we're running a
  command in a multi-command pipeline (pipejob is set) and the
  shell needs to wait for all pipeline commands, i.e.: either the
  time keyword is in use, the SH_MONITOR state is active, or the
  SH_PIPEFAIL option is on.
- case TFIL: Fix the logic for setting job.waitall for the
  non-SH_PIPEFAIL case. Do not 'or' in the boolean value but assign
  it, and include the SH_TIMING (time keyword in use) state too.
- case TTIME: After that fix in case TFIL, we don't need to bother
  setting job.waitall explicitly here.

src/cmd/ksh93/sh.1:
- Add missing documentation for the conditions where the shell
  waits for all pipeline components (time, -o monitor/pipefail).

Resolves: https://github.com/ksh93/ksh/issues/449

NEWS

@@ -3,6 +3,13 @@ For full details, see the git log at: https://github.com/ksh93/ksh/tree/1.0
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2022-06-18:
+
+- Fixed a bug where, with the monitor or pipefail option on, the shell
+  failed to wait for all component commands in a pipeline to terminate if
+  the last component command was an external command and the pipeline was
+  the last command in a background subshell.
+
 2022-06-15:
 
 - Fixed a bug where converting an indexed array into an associative array in