in this change we use global_init_postfork_start() to restart the
log after it is stopped by Preforker.
- rewrite the fork hackery using Preforker helper class
- write "starting ceph client" message to cerr, as the cout was
* fuse_ll.cc: write -1 to signal the parent process that init is
* 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 <firstname.lastname@example.org>