Put the core readlink() code into a separate
_g_win32_readlink_handle_raw() function that takes a file handle,
can optionally ensure NUL-terminatedness of its output
(for cases where we need a NUL-terminator and do *not* need
to get the exact contents of the symlink as it is stored in FS)
and can either fill a caller-provided buffer *or* allocate
its own buffer, and can also read the reparse tag.

Put the rest of readlink() code into separate
functions that do UTF-16<->UTF-8, strip inconvenient prefix
and open/close the symlink file handle as needed.

Split _g_win32_stat_utf16_no_trailing_slashes() into
two functions - the one that takes a filename and the one
that takes a file descriptor. The part of these functions
that would have been duplicate is now split into the
_g_win32_fill_privatestat() funcion.

Add more comments explaining what each function does.
Only g_win32_readlink_utf8(), which is callable from outside
via private function interface, gets a real doc-c...