Cygwin: accounts: Don't keep old schemes when parsing nsswitch.conf

Cygwin: accounts: Don't keep old schemes when parsing nsswitch.conf

The implicit assumption seemed to be that any subsequent occurence of
the same setting in nsswitch.conf is supposed to rewrite the previous
ones completely.  This was not the case if the third or any further
schema was previously defined and the last line defined less than that
(but at least 2), for example:

```
db_home: windows cygwin /myhome/%U
db_home: cygwin desc
```

Let's document this behavior as well.

Signed-off-by: David Macek <david.macek.0@gmail.com>