Allow setting ao backend options via environment

Let libao backend set ao options via TIMIDITY_AO_OPTIONS environment variable.
The variable accepts "key=value" pairs seperated by commans.
For example:
        TIMIDITY_AO_OPTIONS="use_mmap=1,id=1" for ALSA ao backend.
        TIMIDITY_AO_OPTIONS="server=example.com:4713"
etc.

Notes:
        1. Later options have precendance over earlier ones in libao.
        2. Spaces are not trimmed, but supplied as is.
        3. In case of "key=,", an empty string is passed to ao as value.