Revert buggy exec optimisation (re: d6c9821c)

As of 2022-06-18, ksh 93u+m is not capable of being used as /bin/sh
while building GNU binutils. The execution of some of its build
system's dot scripts is incorrectly aborted as an external 'sed'
command is execve(2)'d without forking. This means that incorrect
exec optimization was happening.

Unfortunately I have not been able to derive a minimal reproducer
of the problem yet because the GNU binutils build scripts are very
complex. Pending further research, the optimisation is reverted.
Even if a way to make it work is found, it will not be reintroduced
to the 1.0 branch.

Thanks to @atheik for finding the problem and identifying the
commit that introduced it.

Resolves: https://github.com/ksh93/ksh/issues/507

NEWS

@@ -2,6 +2,13 @@ This documents significant changes in the 1.0 branch of ksh 93u+m.
 For full details, see the git log at: https://github.com/ksh93/ksh/tree/1.0
 Uppercase BUG_* IDs are shell bug IDs as used by the Modernish shell library.
 
+2022-08-05:
+
+- Reverted a buggy exec optimisation introduced on 2022-06-18. It is known
+  to make the build scripts of GNU binutils produce corrupted results.
+
+- Release 1.0.1.
+
 2022-08-01:
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