Improve signal init and shutdown.

Currently all users of the signal subsystem define their own
signal_task structure and initialize the ->fd field from the return
value of para_signal_init(). It is more natural to let the signal
subsystem perform the allocation and the initialization.

This commit renames para_signal_init() to signal_init_or_die() and
changes the function to allocate, initialize and return a signal
task structure (rather than only the file descriptor of the signal
pipe as para_signal_init() did). Similarly, para_signal_shutdown()
is renamed to signal_shutdown() and now takes a pointer to the
signal_task structure which was obtained in an earlier call to
signal_init_or_die(), and frees it. Conversion of all users is
straight forward.

The patch also adds a missing call to signal_shutdown() to audiod.c,
closing an fd leak.