BUG/MINOR: signal: register default handler for SIGINT in signal_init()

BUG/MINOR: signal: register default handler for SIGINT in signal_init()

When haproxy is launched in a background and in a subshell (see example below),
according to POSIX standard (2.11. Signals and Error Handling), it inherits
from the subshell SIG_IGN signal handler for SIGINT and SIGQUIT.

	$ (./haproxy -f env4.cfg &)

So, when haproxy is lanched like this, it doesn't stop upon receiving
the SIGINT. This can be a root cause of some unexpected timeouts, when haproxy
is started under VTest, as VTest sends to the process SIGINT in order to
terminate it. To fix this, let's explicitly register the default signal
handler for the SIGINT in signal_init() initcall.

This should be backported in all stable versions.