Fix type names starting with lowercase 'a' (#69)

Type names that start with a lowercase 'a' cause an error when used:

$ typeset -T al=(typeset bar)
$ al foo=(bar=testset)
/usr/bin/ksh: al: : invalid variable name

The error occurs because when the parser checks for the alias
builtin (to set 'assignment' to two instead of one), only the first
letter of 'argp->argval' is checked (rather than the entire
string). This was fixed in ksh93v- by comparing argp->argval
against "alias", but in ksh93u+m the check can simply be removed
because it is only run when a builtin has the BLT_DCL flag. As of
04b9171, the alias builtin does not have that flag.

src/cmd/ksh93/sh/parse.c:
- Remove the bugged check for the alias builtin.

src/cmd/ksh93/tests/types.sh:
- Add a regression test for type names starting with a lowercase 'a'.

src/cmd/ksh93/tests/types.sh

@@ -642,5 +642,8 @@ bar.foo+=(bam)
 # 'typeset -RF' should not create variables that cause crashes
 "$SHELL" -c 'typeset -RF foo=1; test $foo' || err_exit 'typeset -RF does not work'
 
+# Type names that have 'a' as the first letter should be functional
+"$SHELL" -c 'typeset -T al=(typeset bar); al foo=(bar=testset)' || err_exit "type names that start with 'a' don't work"
+
 # ======
 exit $((Errors<125?Errors:125))