Handle mixers that don't have a playback switch

Some USB soundcards don't provide a master volume. In this case, it is
possible to use an ALSA plugin, 'softvol', that emulates volume in
software.
However, the softvol device lacks a playback switch, and appears as muted
all the time in pnmixer.

This commits fixes that, so that the device appears unmuted, and
clicking on the mute button has no effect.
Fix 'Mixer detach error' when changing sound device

The problem was that the old soundcard name wasn't store anywhere,
therefore pnmixer tried to close the current mixer by using the
name of the new card selected by user.

The variable 'inited' becomes useless in the process.