reallocarray introduction available on glibc 2.26. allocates an array of m*n elements but checking for overflow.

Turns the results of these macros from compound to regular statements using the old `do { ... } while(0)` trick. Must have for function-like macros.

realpath: Fixing undefined behaviour in the second test. If the call fails the resolved_name argument cannot be used for error checking because its state is undefined by SUSv2.
pipe: Changing the order of close and write error handling code. Errors in close could overwrite errno after write errors, returning incorrect error messages.
gmtime: Removed duplicate checks
Other fixes for fseek, rename, mktime, putwchar

This will allow us to redefine the exit function.

For example:
```
#define exit(code) { \
    fprintf(stderr, "%s:%d: exit(%s) in function ‘%s’\n",
        __FILE__, __LINE__, #code, __func__); \
    _exit(code); \
}
```

When the execution reaches the end of the main functions, they implicitly return a successful status.

- Remove incorrect header styles
- Use export.replace where header style was previously needed
- Check compilation of tests using system gcc