several POSIX-compliance issues

variable assignments where the value to be assigned is an empty string don't work, such as
```
foo=
```
redirections can't be applied to brace groups (and perhaps other compound commands):
```
{ :; } >&2
```
the last case item in a case list has to be terminated explicitly, otherwise it won't work:
```
case x in
x) foo
esac
```
case items where the pattern is enclosed in a pair of parens don't work either:
```
case x in
(x) foo ;;
esac
```
pattern matching parameter expansions don't work at all:
```
foo=bar
echo ${foo#}
```
when invoked with -c, the shell doesn't accept positional parameters:
```
nsh -c '' nsh 1 2 3
```

I didn't test any further than this.

nuta / nsh