Fix ${.sh.subshell} counter to actually count level of subshells

This counter is documented as follows:
"The current depth for subshells and command substitution."

But before this commit, the actual behaviour was that the counter
was reset to zero whenever a subshell forked for any reason: a
pipe, background job, running 'ulimit', redirecting stdout in a
command substitution, and more. This behaviour was:

1. Not consistent with the documentation. Non-forked (a.k.a.
   virtual) subshells are an internal implementation detail which
   scripts should not have to be concerned with. The manual page
   doesn't mention them at all.

2. Inherently broken. Since a subshell may fork for any number of
   reasons, even mid-run, and those reasons may change with
   bugfixes and further development, scripts have never actually
   been able to rely on the value of ${.sh.subshell}.

So, this commit fixes the counter to count the levels of all
subshells, both virtual and forked.

src/cmd/ksh93/sh/xec.c: _sh_fork():
- Increase ${.sh.subshell} whenever we fork.

src/cmd/ksh93/sh/subshell.c:
- sh_subfork():
  * Fix comment to properly explain what it does. It doesn't
    "create" a subshell, it forks off an existing virtual subshell.
  * Don't zero ${.sh.subshell}. Instead, since sh_fork() increases
    it but we're forking an existing subshell, undo the increase.
- sh_subshell():
  * Remove 'int16_t subshell' variable. It was unnecessary and
    mostly unused. It was also the wrong type: it was assigned the
    value from shp->subshell which is of type short.
  * Increase and decrease the level of virtual subshells and
    ${.sh.subshell} independently.

src/cmd/ksh93/tests/variables.sh:
- Add regression tests for ${.sh.subshell} in virtual and forked
  subshells of several kinds: comsub, parentheses, pipe, bg job.
- Undo wrong error test count fix from 04b4aef0.

(cherry picked from commit a0e0e29e7e0dbf21e4b3958ae02bde6665fb2696)

src/cmd/ksh93/tests/variables.sh

@@ -621,8 +621,25 @@ function a.set
 	[[ ${.sh.subshell} == 1 ]] || err_exit '${.sh.subshell} should be 1'
 	(
 		[[ ${.sh.subshell} == 2 ]] || err_exit '${.sh.subshell} should be 2'
+		exit $Errors
 	)
 )
+Errors=$?	# ensure error count survives subshell
+
+actual=$(
+	{
+		(
+			echo ${.sh.subshell} | cat	# left element of pipe should increase ${.sh.subshell}
+			echo ${.sh.subshell}
+			ulimit -t unlimited		# fork
+			echo ${.sh.subshell}		# should be same after forking existing virtual subshell
+		)
+		echo ${.sh.subshell}			# a background job should also increase ${.sh.subshell}
+	} & wait "$!"
+	echo ${.sh.subshell}
+)
+expect=$'4\n3\n3\n2\n1'
+[[ $actual == "$expect" ]] || err_exit "\${.sh.subshell} failure (expected $(printf %q "$expect"), got $(printf %q "$actual"))"
 
 set -- {1..32768}
 (( $# == 32768 )) || err_exit "\$# failed -- expected 32768, got $#"
@@ -657,7 +674,7 @@ set --
 
 	exit $Errors
 )
-let "Errors += $?"  # ensure error count survives subshell
+Errors=$?  # ensure error count survives subshell
 
 cd $tmp