Actually the command enable boot-start -user splunk requires a valid shell for the splunk user (the splunk process attempts to run su ).
A workaround is to run enable boot-start and then to add to the file $SPLUNK_HOME/etc/splunk-launch.conf (splunk forwarder 6.1.1)
SPLUNK_OS_USER=splunk
note: this may prevent some functions from the forwarder requiring su or a valid shell (I don't know splunk enough to judge), run at your own risk.
See also: http://installingcats.com/2013/07/30/splunk-account-currently-not-available-boot-start/
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