Cygwin: realpath: fix cygwin installation dir being access via junction

Cygwin: realpath: fix cygwin installation dir being access via junction

Consider this case:

- Cygwin installed in C:\cygwin64

- mklink /j D:\cygwin64 C:\cygwin64

- create testcase calling

    realpath("/", result);
    printf ("%s\n", result);

- start cmd

    >C:\cygwin64\bin\bash -lc <path-to-testcase>
    /
    >D\cygwin64\bin\bash -lc <path-to-testcase>
    /cygdrive/c/cygwin64

This scenario circumventing the mount point handling which is automated
in terms of /, depending on the path returned from GetModuleFileNameW
for the Cygwin DLL.  When calling D:\cygwin64\bin\bash the dir returned
from GetModuleFileNameW is D:\cygwin64\bin, thus root is D:\cygwin64.

However, junctions are treated as symlinks in Cygwin which explains why
the path gets converted to a cygdrive path.

Fix this by calling GetFinalPathNameByHandleW on the result from
GetModuleFileNameW to get the correct root path, even if accessed via
a junction point.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>