global: separate first half of global_init into global_pre_init

global: separate first half of global_init into global_pre_init

The pre_init now captures enough to create the g_ceph_context and parse
and initialize the in-memory config.  However, we don't

 - fiddle with signal handlers
 - init lockdep
 - call config observers (which starts up logging)

Signed-off-by: Sage Weil <sage@inktank.com>