Fix ftype proclamation and definition of ASSERT[-SYMBOL-HOME]-PACKAGE-UNLOCKED

* The ftype proclamation for ASSERT-SYMBOL-HOME-PACKAGE-UNLOCKED could
  be tightened.

* The lambda-list of the ASSERT-SYMBOL-HOME-PACKAGE-UNLOCKED stub in
  src/code/cross-misc.lisp differed from the ftype proclamation and the
  actual implementation. Fixing it exposed the bug fixed in c6f864d.