CLEANUP: stconn: rename final state manipulation functions from cs_* to sc_*

CLEANUP: stconn: rename final state manipulation functions from cs_* to sc_*

This applies the following renaming. It's a bit large but pretty
mechanical:

cs_state -> sc_state  (enum)

cs_alloc_ibuf() -> sc_alloc_ibuf()
cs_is_conn_error() -> sc_is_conn_error()
cs_opposite() -> sc_opposite()
cs_report_error() -> sc_report_error()
cs_set_state() -> sc_set_state()
cs_state_bit() -> sc_state_bit()
cs_state_in() -> sc_state_in()
cs_state_str() -> sc_state_str()