Extended std.cfg about <cstring> functions and added testcases.

I have added a few function definitions to std.cfg and the corresponding test cases.

Note: I also tried to add the following two lines:

```
   <function name="strrchr">  <noreturn>false</noreturn> <leak-igonre/> <arg nr="1"><not-null/><not-uninit/></arg> <arg nr="2"><not-null/><not-uninit/></arg> </function>
   <function name="strpbrk"> <noreturn>false</noreturn> <leak-igonre/> <arg nr="1"><not-null/><not-uninit/></arg> <arg nr="2"><not-null/><not-uninit/></arg> </function>
```

These two lines lead to failing tests in in our testsuite, does anybody know why? I have excluded this lines from this pull request so far, so it can go in.