Use pam_succeed_if with pam_namespace

Use pam_succeed_if to check that the user's shell is set to
/usr/bin/oo-trap-user and only require pam_namespace if it is.

In this manner, we target our polyinstantiation configuration to gear
users only.  The old, less-targetted approach has been causing problems
in some environments where polyinstantiation was not desired for regular
users.

Thanks Dan Trainor for suggesting the pam_succeed_if rule!