sound examples: Add mmap example

This example opens separate OSS capture and playback channels in mmap
mode, places them into a sync group, and starts them together so both
ring buffers advance on the same device timeline. It then monitors the
capture mmap pointer with SNDCTL_DSP_GETIPTR, converts that pointer into
monotonic absolute progress using the reported block count, and copies
newly recorded audio from the input ring to the matching region of the
output ring.

The main loop is driven by an absolute monotonic frame clock rather than
a fixed relative usleep delay. Wakeups are scheduled from the sample
rate using a small frame step similar to the SOSSO timing model, while
the audio path itself stays intentionally simple: just copy input to
output, with no explicit xrun recovery or processing beyond ring
wraparound handling.

MFC after: 1 week
Reviewed by: christos
Differential Revision: https://reviews.freebsd.org/D53749