ao_pulse: add suboption to control buffer size
ao_pulse: lower default buffer size from 1000ms to 250ms

1000ms is a bit insane. It makes behavior on playback speed changes
worse (because the player has to catch up the dropped audio due to
audio-chain reset), and perhaps makes seeking slower.

Note that the problem of playback speed changes misbehaving will be
fixed in the future, but even then we don't want to have a buffer that
large.
terminal: don't initialize termcap etc. if stdout is not a terminal

Otherwise, it seems one of the term* libraries will write escape
sequences to stdout, for whatever reason.