swrap: Abort if socket wrapper directory is too long to be usable

swrap: Abort if socket wrapper directory is too long to be usable

If the socket wrapper directory path is too long to allow reliable
construction of the required Unix domain socket paths then
convert_in_un_alloc() can return ENFILE if paths are truncated in
unfortunate ways.  This can be very hard to debug since, for example,
bind(2) should never return ENFILE.

Instead, abort if the path returned by realpath(3) is unusable.

The code structure is slightly weird but this accommodates an
additional change.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>