typeset: fix short integer restriction (#166)

This commit corrects how shortint was being applied to various
possible typeset variables in error. The short integer option
modifier 'typeset -s' should only be able to be applied if the
the variable is also an integer. Several issues were resolved
with this fix:
- 'typeset -s': created a short integer having an invalid base
  of zero. 'typeset -s foo' created 'typeset -s -i 0 foo=0' and
  now will result in an empty string.
- 'typeset -sL': previously resulted in a segmentation fault.

The following are the various incorrect 'typeset' instances
that have been fixed:

$ 'export foo; typeset -s foo; readonly foo; typeset -p foo'
(before) typeset -x -r -s -i 0 foo=0
( after) typeset -x -r foo

$ 'typeset -sL foo=1*2; typeset -p foo'
(before) Segmentation fault (core dumped)
( after) typeset -L 3 foo='1*2'

$ 'typeset -sR foo=1*2; typeset -p foo'
(before) typeset -s -i foo=2
( after) typeset -R 3 foo='1*2'

$ 'typeset -sZ foo=1*2; typeset -p foo'
(before) typeset -F 0 foo=2
( after) typeset -Z 3 -R 3 foo='1*2'

src/cmd/ksh93/bltins/typeset.c: b_typeset():
- Add conditional check within the 's' option to only
  apply NV_SHORT as well as remove any NV_LONG flag
  if NV_INTEGER flag was set.
- Relocate shortint conditional logic to the 'i' option.

src/cmd/ksh93/tests/attributes.sh:
- Adjust regression tests for '-s' and add '-si' check.

src/cmd/ksh93/tests/attributes.sh

@@ -515,7 +515,8 @@ typeset -A expect=(
 	[i37]='typeset -x -r -i 37 foo'
 	[l]='typeset -x -r -l foo'
 	[n]='typeset -n -r foo'
-	[s]='typeset -x -r -s -i 0 foo=0'
+	[s]='typeset -x -r foo'
+	[si]='typeset -x -r -s -i foo=0'
 	[u]='typeset -x -r -u foo'
 	[A]='typeset -x -r -A foo=()'
 	[C]='typeset -x -r foo=()'