Convert para_fade to subcommands, rename it to para_mixer.

Convert para_fade to subcommands, rename it to para_mixer.

para_fade operates in one of several modes, but the command line
options are shared. So most of the options are ignored for any
particular mode.  With lopsub we can do better and define one
subcommand per mode so that options which are only relevant to one
subcommand can be made local to that subcommand.

With subcommands the syntax of the command changes, which breaks
existing scripts. So it seems to be prudent to rename the executable
from para_fade to para_mixer. The old name was a bit misleading anyway
since the command can do much more than just fade the volume.