Fix crash upon running many subshells (#113)

Co-authored-by: Martijn Dekker <martijn@inlv.org>

An intermittent crash occurred after running many thousands of
virtual/non-forked subshells. One reproducer is a crash in the
shbench fibonacci.ksh test, as documented here:
f3d9e134/bench/fibonacci.ksh (L4-L10)

The apparent cause was the signed and insufficiently large 'short'
data type of 'curenv' and related variables which wrapped around to
a negative number when overflowing. These IDs are necessary for the
'wait' builtin to obtain the exit status from a background job.

This fix is inspired by a patch based on ksh 93v-:
https://build.opensuse.org/package/view_file/shells/ksh/ksh93-longenv.dif?expand=1
https://src.fedoraproject.org/rpms/ksh/blob/f24/f/ksh-20130628-longer.patch

However, we change the type to 'unsigned int' instead of 'long'. On
all remotely modern systems, ints are 32-bit values, and using this
type avoids a performance degradation on 32-bit sytems. Making them
unsigned prevents an overflow to negative values.

src/cmd/ksh93/include/defs.h,
src/cmd/ksh93/include/jobs.h,
src/cmd/ksh93/include/nval.h,
src/cmd/ksh93/include/shell.h:
- Change the types of the static global 'subenv' and the subshell
  structure members 'curenv', 'jobenv', 'subenv', 'p_env' and
  'subshell' to one consistent type, unsigned int.

src/cmd/ksh93/sh/jobs.c,
src/cmd/ksh93/sh/macro.c:
src/cmd/ksh93/sh/name.c:
src/cmd/ksh93/sh/nvtype.c,
src/cmd/ksh93/sh/subshell.c:
- Updates to match new variable types.

src/cmd/ksh93/tests/subshell.sh:
- Show wrong exit status in message on failure of 'wait' builtin.

src/cmd/ksh93/include/nval.h

@@ -75,7 +75,7 @@ struct Namfun
 {
 	const Namdisc_t	*disc;
 	char		nofree;
-	unsigned char	subshell;
+	unsigned int	subshell;
 	uint32_t	dsize;
 	Namfun_t	*next;
 	char		*last;