when unlinkat was called with a link we were removing the target and not
the link itself

In the kernel VFS some file descriptors under /proc/<PID>/fd/ could be
links to a pipe, this is a special case in which the `readlink` function
would not work. So when we detect this case we forward to kernel

when resolving path we don't want always
to resolve also the last components if it is a link. Cause in some cases
we want exactly the path of that link and not the one it points to.

lstat and readlink are some example of functions that don't want to
resolve the last link.

 - avoid error messages on mercury cleanup
 - fix spaces and indentation

relativize path now accepts a file descriptor.

If fd is AT_FDCWD relative path need to be considered with respect to
the current working directory. Otherwise the path is relative to the
directory with the given file descriptor

when the resolved path is the root dir we were returning an empty string

when only PSP are given as input path '//////', we got segfault