af_volume: remove double-negated suboption

You had to use "no-replaygain-noclip" to set this option. Rename it, so
that only one negation is needed.
manpage: af: minor syntax improvements
af_volume: separate softvol volume control from replaygain level

Currently, both replaygain adjustment and user volume control (if
softvol is enabled) share the same variable. Sharing the variable would
cause especially if --volume is used; then the replaygain volume would
always be overwritten.

Now both gain values are simple added right before doing filtering.
af_volume: add detach option

Maybe this should be default. On the other hand, this filter does
something even if the volume is neutral: it clips samples against the
allowed range, should the decoder or a previous filter output garbage.
af_volume: don't print missing replaygain tags as error

There's no reason to. Audio files often lack them.
ao: print (estimated) device buffer size on init in verbose mode