Configurable symlink behaviour

Add a new volumes option 'follow symlinks'. The default setting is
false, symlinks are not followed on the server. This is the same
behaviour as OS X's AFP server.
Setting the option to true causes afpd to follow symlinks on the
server. symlinks may 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.