ceph-fuse: rewrite the fork hackery using Prefork

ceph-fuse: rewrite the fork hackery using Prefork

in this change we use global_init_postfork_start() to restart the
log after it is stopped by Preforker.

* ceph_fuse.c:
   - rewrite the fork hackery using Preforker helper class
   - write "starting ceph client" message to cerr, as the cout was
     closed
     by global_init_postfork_start()
* fuse_ll.cc: write -1 to signal the parent process that init is
  done.
* Preforker.h: add a helper method to return the fd to which, the
  child process can write an int to notify its status.

Signed-off-by: Kefu Chai <kchai@redhat.com>