sepolicy-analyze: Add attribute command.

Add an attribute command to sepolicy-analyze for displaying the list
of types associated with an attribute in a policy.  This is for use
by CTS to check what domains and types are associated with certain
attributes such as mlstrustedsubject and mlstrustedobject.

Change-Id: Ie19361c02feb1ad14ce36862c6aace9e66c422bb
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>