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

NEWS

@@ -3,6 +3,14 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-11-23:
+
+- A bug was fixed that allowed arithmetic expressions to assign out-of-range
+  values to variables of an enumeration type defined with the 'enum' command,
+  causing undefined behavior. Within arithmetic expressions, enumeration
+  values translate to index numbers from 0 to the number of elements minus 1.
+  That range is now checked for. Decimal fractions are ignored.
+
 2021-11-21:
 
 - It is now possible to use types defined by 'enum' in contexts where the