Add `ly:undead?' to predicate list. (issue 93660047)

[dak]

https://codereview.appspot.com/93660047/diff/1/scm/lily.scm
File scm/lily.scm (right):

https://codereview.appspot.com/93660047/diff/1/scm/lily.scm#newcode728
scm/lily.scm:728: (,ly:undead? . "undead object")
Probably more like an "undead container" as the undead thing (to survive
between sessions) is placed inside.

Won't be helpful information to somebody reading the manual which is the
reason I'm somewhat unenthusiastic including it.  On the other hand,
there are lots of predicates sharing that deficiency.

https://codereview.appspot.com/93660047/