Precise `strdup`

[sim642]

Currently strdup (and strndup) return an unknown pointer, which can make things very imprecise. Instead we should:

• [ ] Return a pointer to freshly allocated memory, the same way as malloc.
• [ ] Create a corresponding blob as its value.
• [ ] Blob size should come from the argument string constant or blob size.
• [ ] Blob contents should copy the argument blob contents or summarize the string constant.
• [ ] strndup should additionally only have maximum length n and copy as long prefix.