SIGTT{IN,OU} {in,de}crements worker_processes

SIGTT{IN,OU} {in,de}crements worker_processes

This allows dynamic tuning of the worker_processes count without
having to restart existing ones.  This also allows
worker_processes to be set to a low initial amount in the config
file for low-traffic deployments/upgrades and then scaled up as
the old processes are killed off.

Remove the proposed reexec_worker_processes from TODO since this
is far more flexible and powerful.

This will allow not-yet-existent third-party monitoring tools to
dynamically change and scale worker processes according to site
load without increasing the complexity of Unicorn itself.