IMFS: Allow sockets as generic nodes
IMFS: Improved support for generic nodes

The rtems_filesystem_location_info_t::node_access_2 was unused by the
IMFS.  Use it to hold the context of generic nodes.  This makes it
possible to use node handlers for objects with and without a
corresponding file system node.

For example network sockets created with socket() have only a file
descriptor, but no corresponding file system node.  The UNIX(4) domain
sockets can be bound to file system nodes.  In both cases the
rtems_filesystem_location_info_t must provide a pointer to the socket
structure used by the socket node handlers.  With the context pointer
(for sockets, this is the socket structure) in
rtems_filesystem_location_info_t::node_access_2 the same node handlers
can be used in both cases.