CLEANUP: stconn: rename cs_{i,o}{b,c} to sc_{i,o}{b,c}

CLEANUP: stconn: rename cs_{i,o}{b,c} to sc_{i,o}{b,c}

We're starting to propagate the stream connector's new name through the
API. Most call places of these functions that retrieve the channel or its
buffer are in applets. The local variable names are not changed in order
to keep the changes small and reviewable. There were ~92 uses of cs_ic(),
~96 of cs_oc() (due to co_get*() being less factorizable than ci_put*),
and ~5 accesses to the buffer itself.