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>
ceph-conf: use global_pre_init to avoid starting logging

This avoids starting up logging, which is not appropriate when we are
examining the config state and not actually starting up the entity in
question.

Fixes: #7849
Signed-off-by: Sage Weil <sage@inktank.com>