This commit adds a thread safe MEMORY keytab implementation

	This commit adds a thread safe MEMORY keytab implementation
	that is compatible with Heimdal 0.7.  Each successful resolve
	returns a handle to a keytab and increases the internal
	reference count.  Each close invalidates the handle and
	decreases the reference count.  When the reference count hits
	zero, the keytab is destroyed.

	When a kt_destroy function is added, the automatic destruction
	after close behavior will be removed.

ticket: 5411

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@19126 dc483132-0cff-0310-8789-dd5450dbe970