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)

NEWS

@@ -15,6 +15,10 @@ Any uppercase BUG_* names are modernish shell bug IDs.
   macOS, by eliminating an invalid/undefined use of memccpy() on overlapping
   buffers in the commonly used sfputr() function.
 
+- Fix the ${.sh.subshell} level counter; it is no longer reset to zero when a
+  non-forked subshell happens to fork into a separate process for some reason
+  (an internal implementation detail that should be unnoticeable to scripts).
+
 2020-06-04:
 
 - Fix BUG_KBGPID: the $! special parameter was not set if a background job