Changes

osmo_io: Lazy-initialize during osmo_iofd_setup()

This way new threads (!=main) don't need to explicitly call
osmo_iofd_init() explicitly before using osmo_io. This is specially
useful for threads using osmo_io indirectly, eg, when logging.

The osmo_iofd_setup() API is really the only main API which requires
initialization. All other APIs requiring it come after osmo_iofd_setup()
since they use an osmo_iofd pointer.

Change-Id: Ia543c1b9d111c929cd9dbed266b6d21b74e47e4b