Minor changes in driver description

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Update documentation with h command

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Don't call deinitialization from signal handler

Eugene Toder reports:

When I abort the playback using Ctrl-C I get the following error:

Assertion 'pthread_mutex_destroy(&m->mutex) == 0' failed at
pulsecore/mutex-posix.c:83, function pa_mutex_free(). Aborting.
Aborted (core dumped)

This happens because xmp registers a signal handler for SIGINT that calls
sound->deinit(). For the PulseAudio driver this calls pa_simple_free(),
which is apparently not happy to be called from a signal handler. Commenting
out the call to deinit() fixes the problem, and seems to work well for both
OSS and PulseAudio.

Generally speaking, calling deinit() from a signal handler is dangerous --
it's likely to call free() or similar, which is not signal safe. Is this
necessary? If the idea is to have a graceful shutdown on Ctrl-C, the handler
should set a flag or use longjmp() to exit the play loop in main(). Also,
attempting a graceful shutdown for SIGFPE and especially SIGSEGV is probably
not a good idea.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>