Don't modify SIGCHLD handler when using async mode

[hoytech]

• When I am using async mode and the EV event loop, I wasn't being
  notified of process exits via the condition variable returned by
  AnyEvent::Util::run_cmd(... $ssh->make_remote_command ...).
• After applying this patch, I am immediately notified when (for example)
  the ssh connection is lost.
• I believe that the other locations in the file that install SIGCHLD
  handlers would also break EV's child handlers, but this doesn't affect
  my use-case since after the SSH object is constructed I only use
  $ssh->make_remote_command().

[salva]

I have applied your change with some additional checks for the deactivation of the $SIG{CHLD} localization when it is already been used by the main program.

local $SIG{CHLD} = sub {} is just an ugly hack used to break from restartable syscalls.