Root directory being a junction fools realpath()

[David Macek]

Greetings.

I think I found a way to fool Cygwin into misplacing its root
directory. Given recent Cygwin with GCC and a simple test program:

```
#include <stdio.h>
#include <limits.h>
#include <stdlib.h>
int main() {
char real[PATH_MAX];
realpath("/", real);
printf("/ -> %s\n", real);
return 0;
}
```

... compiled with:

$ gcc -D_DEFAULT_SOURCE -o new.exe -Wall -Wextra -std=c11 new.c

... and my set-up with a junction (`mklink /j link target`) realpath()
result is dependent on which path I use to invoke the Cygwin root
process:

> dir C:\ | findstr cygwin64
10. 02. 2021  12:13    <DIR>          cygwin64

> dir D:\ | findstr cygwin64
14. 02. 2021  12:19    <JUNCTION>     cygwin64 [C:\cygwin64]

> C:\cygwin64\bin\bash -lc /cygdrive/w/new
/ -> /

> D:\cygwin64\bin\bash -lc /cygdrive/w/new
/ -> /cygdrive/c/cygwin64

It seems the current working directory doesn't matter at all, same as
the directory where the test program resides. Regular substs don't
seem to have the same effect. I can do more tests if requested.

I assume this is not intended behavior, because the root should always
be the root for Cygwin processes, no matter how they're invoked.

If possible ping me on IRC if anything because I don't check my mail too often.

-- 
David Macek