GHC.Conc.ensureIOManagerIsRunning now creates an IO manager thread if
one does not exist or has died/exited.

Unfortunately this exposed a problem caused by the fact that we have
two base packages, and hence two IO managers, in GHCi: see NOTE
[io-manager-ghci] in rts/Linker.c.  The workaround can go away if/when
we switch to a dynamically linked GHCi.