Clean up obsolere test(1) -a option/operator - use sh && instead.
Take the opportunity to do a little sh formatting cleanups, the \
line continuation can ususually be avoided by placing sh operators at
the appropriate places.

Note the [ x"$var" = x"$other" ] usage no longer needs the 'x',
avoiding that kind of nonsense is partly why test was much more
precisely specified, and a lot of the unnecessary stuff made obsolete.
Those however I did not remove (they're harmless).

Don't specify a volume alias if none is given instead of using a default
name. Otherwise iscsictl fails for using non-unique names if more than
one volume is used.

Handle missing auth file.
Skip empty lines.

If /etc/iscsi/volumes does not exist, explicitly exit with 0 status

Add start script to attach iscsi volumes at boot.

The default is to execute the script (iscsid_volumes=YES), so if you have
any volumes defined, you should also start iscsid (iscsid=YES) to avoid
error messages.