Because I've fixed some additional bugs and added new tests and
rebased on recent master branch, I'm reposting the entire nofork (mksh
${|...} and ksh ${ ... }) substitution diffs rather than do another
incremental patch.  To make the individual patches a more reasonable
size, I've split the C code and doc from the tests.

These diffs now rely on the change to input.c posted in workers/52153
to properly handle trailing newlines in the output captured by ${ ...
}.