alsa: New writer option: --buffer-time.

This removes the old way of setting of private_alsa_write_data->buffer_time

Currently we set the buffer time to the minimum of the maximum
buffer time and 500ms. This results in different values for different
hardware. In view of the introduction of the sync filter it seems to
be desirable to have more control over this value.

This patch adds the --buffer-time config option to the alsa
writer which allows to specify a suitable value, the default being
170ms. The unit of private_alsa_write_data->buffer_time is changed
from microseconds to milliseconds as this avoids to divide by 1000
in ->pre_select().