CLEANUP: check: rename all occurrences of stconn "cs" to "sc"

CLEANUP: check: rename all occurrences of stconn "cs" to "sc"

The check struct had a "cs" field renamed to "sc", which also required
a tiny update to a few functions using it to distinguish a check from
a stream (log.c, payload.c, ssl_sample.c, tcp_sample.c, tcpcheck.c,
connection.c).

Function arguments and local variables called "cs" were renamed to "sc".
The presence of one "cs=" in the debugging traces was also turned to
"sc=" for consistency.