initialize signal handlers before writing pid file

initialize signal handlers before writing pid file

This prevents trigger-happy init scripts from reading the pid
file (and thus sending signals) to a not-fully initialized
master process to handle them.

This does NOT fix anything if other processes are sending
signals prematurely without relying on the presence of the pid
file.  It's not possible to prevent all cases of this in one
process, even in a purely C application, so we won't bother
trying.

We continue to always defer signal handling to the main loop
anyways, and signals sent to the master process will be
deferred/ignored until Unicorn::HttpServer#join is run.