[MEDIUM] convert all signals to asynchronous signals

[MEDIUM] convert all signals to asynchronous signals

The small list of signals currently handled by haproxy were processed
as soon as they were received. This has caused trouble with calls to
pool_gc2() occuring in the middle of libc's memory management functions
seldom causing deadlocks preventing the old process from leaving.

Now these signals use the new async signal framework and are called
asynchronously, when there is no risk of recursion. This ensures more
reliable operation, especially for sensible processing such as memory
management.