Add LOCK_CLOSURE macro for use in C--, which inlines the capability check.

This patch also tweaks lockClosure to be INLINE_HEADER, so C-- clients
don't accidentally use them and updates some other code which locks closures
to do the capability check.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>
Don't call dirty_MVAR on atomicReadMVar unless we change the MVar.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>
Implement tryAtomicReadMVar#.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>
Release note for atomicReadMVar and friends.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>
Fix typo in description