Make jboss a system user without a login shell

Instead of creating jboss as uid>=500, stomping on regular users, create
it as uid 254 (by convention as specified at [1]), and adjust the init
script according to RHBZ #678671 [2] to use a temporary shell for JBoss
invocation.

[1] http://slackbuilds.org/uid_gid.txt
[2] https://bugzilla.redhat.com/show_bug.cgi?id=678671