MEDIUM: unblock signals on startup.

MEDIUM: unblock signals on startup.

A problem was reported recently by some users of programs compiled
with Go 1.5 which by default blocks all signals before executing
processes, resulting in haproxy not receiving SIGUSR1 or even SIGTERM,
causing lots of zombie processes.

This problem was apparently observed by users of consul and kubernetes
(at least).

This patch is a workaround for this issue. It consists in unblocking
all signals on startup. Since they're normally not blocked in a regular
shell, it ensures haproxy always starts under the same conditions.

Quite useful information reported by both Matti Savolainen and REN
Xiaolei actually helped find the root cause of this problem and this
workaround. Thanks to them for this.

This patch must be backported to 1.6 and 1.5 where the problem is
observed.