This patch has the following changes:

  * Check return code on close commands.

  * Fixes a bug where NODOTFILES didn't detect a dot if the first
    character was a slash and the second a dot.

  * twoftpd-auth recognizes a new environment variable TWOFTPD_NOPASS.
    This variable may contain a whitespace-seperated list of users who
    we should bother to ask a password of, but should instead just
    authenticate them immediately with no password. 

  * A new FTP server is created, twoftpd-drop.  It is a write-only
    server, with a special STOR function which will try to generate a
    new filename if the requested one is already used, by appending _
    then a number.

  * A few static items in store.c are made not-so-static, so that we
    can use them in twoftpd-drop.  It might be cleaner to move
    drop_handle_stor into store.c instead, but then again it might not
    be.