auto connect all ports in alsa_seq MIDI driver
auto connect all ports in alsa_seq MIDI driver

Context: ticket #414

The alsa_seq driver creates multiple ports (N/16) for
N="synth.midi-channels", each mapped to synth channels: port*16+channel.
This is already implemented.

When the "midi.autoconnect" is specified, instead of subscribing only
the last created port, all ports are sequencially subscribed to each
available external MIDI inputs. When the last port has been subscribed,
the next subscription starts again from port 0.

FluidSynth/fluidsynth,FluidSynth/fluidsynth,FluidSynth/fluidsynth,FluidSynth/fluidsynth