Fixed bug in pand that was probably also a bug in por, pargs, and plet.

Quoting :doc note-7-0

  (ACL2(p) and ACL2(hp) only) We fixed a bug in [pand], which we
  believe was also a bug in [por], [pargs], and [plet]. (Technical
  remark: this bug had a low-level cause, pertaining to maintenance
  of a Lisp variable, *ld-level*, that holds the number of nested
  calls of [ld].) Thanks to Jared Davis for reporting this bug and to
  David Rager for assisting in its repair. Below is a slightly
  simplified version of Jared's example, which formerly caused an
  assertion error but now works as expected.

    (defmacro pand* (x y)
      `(spec-mv-let (yval)
                    ,y
                    (mv?-let (xval)
                             ,x
                             (if xval
                                 yval
                               nil))))
    (defn f3 (x)
      (pand* x x))
    (defn f4 (x)
      (pand x
            (f3 x)))
    (value-triple (f4 5))