threading: support more than nthreads at runtime

Hook a couple functions (notably cfunction) to handle adopting
foreign threads automatically when used.

n.b. If returning an object pointer, we do not gc_unsafe_leave
afterwards as that would render the pointer invalid. However, this means
that it can be a long time before the next safepoint (if ever). We
should look into ways of improving this bad situation, such as pinning
only that specific object temporarily.

n.b. There are some remaining issues to clean up. For example, we may
trap pages in the ptls after GC to keep them "warm", and trap other
pages in the unwind buffer, etc.

Author: vtjnash

test/distributed_exec.jl

@@ -153,12 +153,12 @@ function _getenv_include_thread_unsafe()
 end
 const _env_include_thread_unsafe = _getenv_include_thread_unsafe()
 function include_thread_unsafe_tests()
-    if Threads.nthreads() > 1
+    if Threads.maxthreadid() > 1
         if _env_include_thread_unsafe
             return true
         end
-        msg = "Skipping a thread-unsafe test because `Threads.nthreads() > 1`"
-        @warn msg Threads.nthreads()
+        msg = "Skipping a thread-unsafe test because `Threads.maxthreadid() > 1`"
+        @warn msg Threads.maxthreadid()
         Test.@test_broken false
         return false
     end