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.
Fix hotkeys in prefs when unset

If hotkeys are not set, the label is (None). When gtk_accelerator parses
it, it fails and set keysym to zero. In this case, we shouldn't feed
that to XKeysymToKeycode(), since it will indeed return a value (204)
which will be saved in the prefs.
Add a (geek) way to reset hotkeys

Maybe using <Ctrl>C is not very user friendly, but it's easy and
intuitive to me.

TODO: translation for the new line in the UI
Grey out hotkey volume step when disabled

Also rename hotkey_spin in hotkey_vol_spin for consistency with
hotkey_vol_label.
Explicit initialization of static variable

Follow the 'Say What You Mean' rule.
Merge pull request #35 from elboulangero/master

Various little fixes