0.8.12.7: Merge package locks, AKA "what can go wrong with a 3783 line patch?"

0.8.12.7: Merge package locks, AKA "what can go wrong with a 3783 line patch?"
          ... Controlled by the presence of :sb-package-locks in target
                 features.
          ... This builds both with and without package locks on both
                 x86 Linux and SunOS Sparc, with both CMUCL and SBCL
                 as host -- so chances are it should build elsewhere as
                 well.
          ... Remaining TODO: turn package locking errors from lexical
                 constructs to program errors in the produced code, fix
                 the bits in SBCL that hit host's SBCL-tyle package locks
                 (relevant FIXME is in src/cold/shared.lisp).