Configurable symlink behaviour

Add a new volumes option 'followsymlinks' which modifies Netatalk's
handling of symlinks. Without the option (default behaviour) symlinks
are not followed by afpd.
Setting the volume option causes afpd to follow any symlink on the
server. symlinks may then point outside of the AFP volume, currently
afpd doesn't do any checks for "wide symlinks".

In order to provide a somewhat consistent interface, add an API
of o* function wrappers for filesytem related functions.
These functions take an an additonal option arg (hence the o) which
tells the functions whether symlinks shall be followed or not.