arith: implement range checking for enum types

Within arithmetic expressions, enumeration values of variables of a
type created with the 'enum' command translate to index numbers
from 0 to the number of elements minus 1. However, there was no
range checking on this in the arithmetic subsystem, allowing the
assignment of out-of-range values that did not correspond to any
enumeration value.

Variables of an enum type are internally unsigned short integers
(NV_UINT16), like those created with 'integer -su', except with an
additional discipline function (ENUM_disc).

src/cmd/ksh93/bltins/enum.c,
src/cmd/ksh93/include/builtins.h:
- To implement range checking, the arithmetic system needs access
  to the 'nelem' (number of elements) member of 'struct Enum'. This
  is only defined locally in enum.c. We could move that to name.h
  so arith.c can access it, but enum.c has code that supports
  compiling as standalone. So, instead, define a quick extern
  function, b_enum_elem(), that does the necessary type conversion
  and returns a type's number of elements.
- Add --man documentation for the arithmetic subsystem behaviour
  for enum types. Tell the enuminfo() function, which dynamically
  inserts values into the documentation, how to process new \f tags
  'lastv' (the last-defined value) and 'lastn' (the number of the
  last element).

src/cmd/ksh93/sh/arith.c: arith():
- For NV_UINT16 variables with an ENUM_disc discipline, check the
  range using b_enum_elem() and error out if necessary.

Resolves: https://github.com/ksh93/ksh/issues/335

src/cmd/ksh93/bltins/enum.c

@@ -21,8 +21,10 @@
 #pragma prototyped
 #include	"defs.h"
 
+#define ENUM_ID "enum (ksh 93u+m) 2021-11-23"
+
 const char sh_optenum[] =
-"[-?@(#)$Id: enum (ksh 93u+m) 2021-11-21 $\n]"
+"[-?@(#)$Id: " ENUM_ID " $\n]"
 "[--catalog?" ERROR_CATALOG "]"
 "[+NAME?enum - create an enumeration type]"
 "[+DESCRIPTION?\benum\b is a declaration command that creates an enumeration "
@@ -30,6 +32,7 @@ const char sh_optenum[] =
     "array variable \atypename\a.]"
 "[+?If the list of \avalue\as is omitted, then \atypename\a must name an "
     "indexed array variable with at least two elements.]" 
+"[+?For more information, see \atypename\a \b--man\b.]"
 "[i:ignorecase?The values are case insensitive.]"
 "\n"
 "\n\atypename\a[\b=(\b \avalue\a ... \b)\b]\n"
@@ -43,15 +46,19 @@ const char sh_optenum[] =
 ;
 
 static const char enum_type[] =
-"[-1c?\n@(#)$Id: type (ksh 93u+m) 2021-11-21 $\n]"
+"[-?@(#)$Id: " ENUM_ID " $\n]"
 "[--catalog?" ERROR_CATALOG "]"
 "[+NAME?\f?\f - create an instance of type \b\f?\f\b]"
 "[+DESCRIPTION?The \b\f?\f\b declaration command creates a variable for "
     "each \aname\a with enumeration type \b\f?\f\b, a type that has been "
     "created with the \benum\b(1) command.]"
 "[+?The variable can have one of the following values: \fvalues\f. "
-    "The values are \fcase\fcase sensitive.]"
-"[+?If \b=\b\avalue\a is omitted, the default is \fdefault\f.]"
+	"The values are \fcase\fcase sensitive. "
+	"If \b=\b\avalue\a is omitted, the default is \fdefault\f.]"
+"[+?Within arithmetic expressions, these values translate to index numbers "
+	"from \b0\b (for \fdefault\f) to \flastn\f (for \flastv\f). "
+	"It is an error for an arithmetic expression to assign a value "
+	"outside of that range. Decimal fractions are ignored.]"
 "[+?If no \aname\as are specified then the names and values of all "
         "variables of this type are written to standard output.]"
 "[+?\b\f?\f\b is built in to the shell as a declaration command so that "
@@ -90,6 +97,14 @@ struct Enum
 	const char	*values[1];
 };
 
+/*
+ * For range checking in arith.c
+ */
+short b_enum_nelem(Namfun_t *fp)
+{
+	return(((struct Enum *)fp)->nelem);
+}
+
 static int enuminfo(Opt_t* op, Sfio_t *out, const char *str, Optdisc_t *fp)
 {
 	Namval_t	*np;
@@ -102,6 +117,16 @@ static int enuminfo(Opt_t* op, Sfio_t *out, const char *str, Optdisc_t *fp)
 		return(0);
 	if(strcmp(str,"default")==0)
 		sfprintf(out,"\b%s\b",ep->values[0]);
+	else if(memcmp(str,"last",4)==0)
+	{
+		while(ep->values[++n])
+			;
+		n--;
+		if(str[4]=='v')
+			sfprintf(out,"\b%s\b",ep->values[n]);
+		else
+			sfprintf(out,"\b%d\b",n);
+	}
 	else if(strcmp(str,"case")==0)
 	{
 		if(ep->iflag)