basic shell command without arguments doesn't work in posix

a simple shell command like `echo 1` doesn't work in posix.

test:
```
#include

namespace bp = boost::process;

int main() {
return bp::system("echo 1", bp::shell);
}
```

this is a workaround:
```
#include

namespace bp = boost::process;

int main() {
return bp::system(bp::shell().string(), "-c", "echo 1");
}
```

I have a fix that I will submit as a pull request.