alsa_in/out: Convert between sample rates when necessary

[mildsunrise]

alsa_in and alsa_out set the playback latency of their ports to the target delay. The problem is the target delay is in terms of the ALSA sample rate, so it should be converted to JACK's sample rate.

Example: Imagine JACK is running at 48 kHz, and alsa_out is invoked like:

alsa_out -r 96000 -t 512

Currently, alsa_out will report 512 frames of playback latency. After the fix, it converts to 48kHz and correctly reports 256.

Also converts the result of jack_frames_since_cycle_start to ALSA sample rate.