Change prctl calls from `-t basic` to `-t system` for SmartOS

Newer global zones of SmartOS changed the behavior of `prctl`.
The man pages still show `-t basic` as being a valid flag, but
a user reported it no longer worked in practice in #117.  This
change fixes that issue according to Joyent's recommendation.

I tested this on datasets 1.6, 1.8, 13.1 by installing the packages, starting with `svcadm enable -r riak`, attaching via `sudo riak attach` and checking `os:cmd('ulimit -n').` which printed the correct value.