Pass raw "float" volume to OpenAL

The client uses float values between 0.0 and 1.0 to represent the volume
of sound samples. This is the range required by OpenAL. But the generic
part of the sound system multiplied the raw float value with 374 and
clamped it to a full integer. That worked by luck withth the OpenAL
backend but broke at least the silencer powerup. Solve this problem by
adding a new field "float oal_vol" to the channel_t struct and use it to
pass the raw float value to OpenAL.

This fixes issue #18
Calculate the AL_GAIN by s_volume and don't set a global AL_GAIN
Add a cvar "s_openal_maxgain" to clamp AL_GAIN

This cvar is a last resort if all other measures to prevent overdriven
preamplifation fail. Setting it to lower value than 1.0 limits the
overall dynamic range, so sound quality is lost. This is especially
hearable when low volume samples are encountered, like the shotgun
combined with the silencer.
Reduce default volume to 0.3 on non-linux-platforms

If the volume is set too high the OpenAL backend preamplification leads
to overdriven sound samples. It's not quite understandable to me why
that only happen on platforms other than Linux (maybe a bug in OpenAL?)
and there's not much we can do against it besides reducing the volume.
As the side note: Simmilar behavior can be seen at least in ioQ3 and
dhewm3...