Send optional AFP messages for vetoed files

New option "veto messages" can be used to enable sending messages.
Then whenever a client tries to access any file or directory
with a vetoed name, it will be sent an AFP message indicating
the name and the directory.

Feature request #81.
New boolean volume option "delete veto files"

If this option is set to yes, then Netatalk will attempt to
recursively delete any vetoed files and directories.

Remove obselete code that tries to delete dangling symlinks, this
is not necessary anymore since Netatalk support UNIX symlinks.

Modify behaviour of the VFS function deletecurdir of the "ea" VFS
module so it deletes all files beginning with "._".

From feature request #82.
systemd unit dir is /usr/lib/systemd/system now
FPExchangeFiles must only switch data file, not metadata

In Netatalk 3, afp_exchangefiles() moves the metadata EAs togehter with
the files, so we must copy the metadata EAs back at then end.

Fixes bug #521.
Remove obsolete variable
Prepare 3.0.5 release
Bump version