CLEANUP: mux-fcgi: rename all occurrences of stconn "cs" to "sc"

Function arguments and local variables called "cs" were renamed to
"sc" to avoid future confusion. There were also 3 places in debugging
traces where "cs" used to display the stconn, which were turned to "sc"
for similar reasons. The number of streams "nb_cs" was turned to "nb_sc".
1 file changed