Introduce afh_get_afhi_txt().

Introduce afh_get_afhi_txt().

The afh receiver needs a pretty-printed version of the audio format
handler structure, like the one that is printed in print_info()
of afh.c.

This commit moves print_info() to afh_common.c and makes it print
into a buffer rather than to stdout. The function is renamed to the
more descriptive afh_get_afhi_txt().

The single caller in afh.c is changed accordingly.