Use automatic variable in Makefile

As written, the $(init_SCRIPTS) rule in etc/init.d/Makefule.am
would not work as expected if the init_SCRIPTS variable were
to contain any elements other than zfs.  Fix this by replacing
the hard-coded 'zfs' reference with $@.

Signed-off-by: Ned Bass <bass6@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #410