silence: fix issues related to number of channels
silence: fix issues related to number of channels

Within the silence effect, convert properly between wide samples and
non-wide samples counts, and document which is which (and where).
Should solve some, but not all of the reported issues about silence
(like stopping too early on multi-channel input).

MageSlayer/sox,Motiejus/sox,mhartzel/sox_personal_fork,CaptainHayashi/sox,Motiejus/sox,mhartzel/sox_personal_fork,jacksonh/sox,Distrotech/sox,pcqpcq/sox,MageSlayer/sox,cbagwell/sox,MageSlayer/sox,davel/sox,cbagwell/sox,CaptainHayashi/sox,MageSlayer/sox,davel/sox,Motiejus/sox,cbagwell/sox,jacksonh/sox,Motiejus/sox,Distrotech/sox,MageSlayer/sox,mhartzel/sox_personal_fork,cbagwell/sox,uklauer/sox,uklauer/sox,mhartzel/sox_personal_fork,davel/sox,CaptainHayashi/sox,Distrotech/sox,pcqpcq/sox,jacksonh/sox,CaptainHayashi/sox,Distrotech/sox,pcqpcq/sox,davel/sox,jacksonh/sox,uklauer/sox,pcqpcq/sox,uklauer/sox