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/sh/subshell.c

@@ -103,7 +103,7 @@ static struct subshell
 	char		pwdclose;
 } *subshell_data;
 
-static int subenv;
+static unsigned int subenv;
 
 
 /*
@@ -176,7 +176,7 @@ void sh_subfork(void)
 {
 	register struct subshell *sp = subshell_data;
 	Shell_t	*shp = sp->shp;
-	int	curenv = shp->curenv;
+	unsigned int curenv = shp->curenv;
 	pid_t pid;
 	char *trap = shp->st.trapcom[0];
 	if(trap)
@@ -251,7 +251,7 @@ Namval_t *sh_assignok(register Namval_t *np,int add)
 	Dt_t			*dp= shp->var_tree;
 	Namval_t		*mpnext;
 	Namarr_t		*ap;
-	int			save;
+	unsigned int		save;
 	/* don't bother to save if in a ${ subshare; } */
 	if(sp->subshare)
 		return(np);
@@ -456,7 +456,7 @@ Sfio_t *sh_subshell(Shell_t *shp,Shnode_t *t, volatile int flags, int comsub)
 	struct subshell sub_data;
 	register struct subshell *sp = &sub_data;
 	int jmpval,nsig=0,duped=0;
-	int savecurenv = shp->curenv;
+	unsigned int savecurenv = shp->curenv;
 	int savejobpgid = job.curpgid;
 	int *saveexitval = job.exitval;
 	char *savsig;