stc/libadacl.c: use a "cleanup" routine in acl_update_entry().

This is in preparation for comparing the qualifiers of the given and
existing ACL entries. Since acl_get_qualifier() can allocate memory,
we need to be sure that memory gets freed, even if an error occurs. A
"cleanup" routine and liberal use of "goto" is the standard pattern
throughout the rest of the library to deal with that problem.