1.0.7.14: thread-safe INTERN, EXPORT, &co

1.0.7.14: thread-safe INTERN, EXPORT, &co

 * Modifications to packages grab a global lock. INTERN is the only
   real potential performance bottleneck here, but as long as the
   symbol already exists it doesn't need to get the lock.

   We need a global lock instead of a per-package lock because eg.
   (EXPORT 'FOO::BAR :FOO) and (INTERN "BAR" :ZOT) can conflict, even
   though they operate on different packages.

   Since races should be rare we use a spinlock to avoid making a
   system call for every release.

   Interrupt safety? Probably no. It's likely that you can wedge the
   package system into a bad state if you really try.