Returns an existing (predefined typed) attribute or create one if parameter create is true.
If parameter create is true and it doesn't exist yet it will be created (based
upon its SecurityAttributeType) first, but only if the SecurityAttributes itself
isn't readOnly (then a AttributesReadOnlyException will be thrown).
If parameter create is false and it doesn't exist yet a NULL value will be
returned.
If there is no SecurityAttributeType defined for the attribute (name), a new attribute
with INFO_CATEGORY will be created.
Parameters:
name - name of a predefined SecurityAttributeType (for this JetspeedPrincipal type)
create - add the attribute when it doesn't exist yet
Returns:
an existing attribute or one created on the fly (if parameter create is true)