Updates for base-4.7.0 (GHC 7.8)

This adds the new `withMVarMasked` and `tryReadMVar` functions.  They seem to work in GHCI, but I haven't tested them extensively.

This also changes which modules are exported for GHC 7.6 - as `Control.Concurrent.QSem` and `Control.Concurrent.QSemN` weren't being exported - even though they haven't been removed from any published version of base.