MEDIUM: global: add a 'hard-stop-after' option to cap the soft-stop time

MEDIUM: global: add a 'hard-stop-after' option to cap the soft-stop time

When SIGUSR1 is received, haproxy enters in soft-stop and quits when no
connection remains.
It can happen that the instance remains alive for a long time, depending
on timeouts and traffic. This option ensures that soft-stop won't run
for too long.

Example:
  global
    hard-stop-after 30s  # Once in soft-stop, the instance will remain
                         # alive for at most 30 seconds.