decoder/sndfile: add sndfile_sample_format()

Prepare for other sample formats.
decoder/sndfile: move sf_readf_int() call to sndfile_read_frames()
decoder/sndfile: support float and 16 bit samples

Support these PCM formats natively, instead of letting libsndfile
convert everything to 32 bit.
decoder/sndfile: log detailed sf_open_virtual() error message
doc/user: more NFS instructions
doc/user: add section "Satellite setup"