Merge pull request #10 from kazu-yamamoto/kazu-reimp5

Prevent a warning.
ioManagerCapabilitiesChanged takes no arguments and instead gets the current enabled number of capabilities by executing getNumCapabilities after it acquires a lock on the IO managers array.
Use (.&.) instead of mod in GHC.Event.Manager since the modulus is a power of 2.
Add some commentary in GHC.Event.Manager to the explain the poll loop.
Added UNPACK to emLock in GHC.Event.Manager Manager datatype.
Added NOINLINE directive to the numEnabledEventManagers IORef in GHC.Event.Thread.