Add error message for ambiguous long-form option abbreviation

Before:

  $ set -o hist
  -ksh: set: hist: bad option(s)

After:

  $ set --hist
  -ksh: set: hist: ambiguous option

In ksh 93u+m, there are three options starting with 'hist', so the
abbreviation does not represent a single option. It is useful to
communicate this in the error message.

In addition, "bad option(s)" was changed to "unknown option", the
same message as that given by AST optget(3), for consistency.

src/cmd/ksh93/sh/string.c:
- Make sh_lookopt() return -1 for an ambiguous option. This is
  trivial as there is already an 'amb' variable flagging that up.

src/cmd/ksh93/sh/args.c:
- Use the negative sh_lookopt() return status to decide between
  "unknown option" and "ambiguous option".

src/cmd/ksh93/data/builtins.c: sh_set[]:
- Explain the --option and --nooption forms in addition to the -o
  option and +o option forms.
- Document the long options without their 'no' prefixes (e.g. glob
  instead of noglob) as this simplifies documentation and the
  relation with the short options makes more sense. Those names are
  also how they show up in 'set -o' output and it is better for
  those to match.
- Tweaks.

src/cmd/ksh93/tests/basic.sh

@@ -905,7 +905,7 @@ done
 # ksh2020 regression: https://github.com/att/ast/issues/1284
 actual=$($SHELL --verson 2>&1)
 actual_status=$?
-expect=': verson: bad option(s)'
+expect=': verson: @(bad option(s)|unknown option)'
 expect_status=2
 [[ "$actual" == *${expect}* ]] || err_exit "failed to handle invalid flag" \
 	"(expected *$(printf %q "$expect")*, got $(printf %q "$actual"))"