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/include/version.h

@@ -21,7 +21,7 @@
 
 #define SH_RELEASE_FORK	"93u+m"		/* only change if you develop a new ksh93 fork */
 #define SH_RELEASE_SVER	"1.0.0-beta.2"	/* semantic version number: https://semver.org */
-#define SH_RELEASE_DATE	"2021-11-21"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_DATE	"2021-11-23"	/* must be in this format for $((.sh.version)) */
 #define SH_RELEASE_CPYR	"(c) 2020-2021 Contributors to ksh " SH_RELEASE_FORK
 
 /* Scripts sometimes field-split ${.sh.version}, so don't change amount of whitespace. */