SmartOS support

Changes to allow support of Joyent's packaged nginx on SmartOS:

params.pp
Added variable $solaris_nx_daemon_user to toggle between 'www' and 'webservd'.
Added selector to determine $nx_conf_dir value based on $::kernelversion.

nginx.conf.erb
Replaced hardcoded path with scope.lookupvar for nx_conf_dir.