Set the default message size for SCTP to 64KB.

This makes SCTP with default parameters work on CentOS 6; formerly
it was just using the TCP default (128KB) and failing with
a "message too long" error.  It might be possible to fix this with
some manipulation of other default values, so that TCP and SCTP
can use the same default message size, but I haven't figure out
what this would be.

This ties up one loose end from Issue 131.

src/iperf_api.c

@@ -800,6 +800,8 @@ iperf_parse_arguments(struct iperf_test *test, int argc, char **argv)
     if (blksize == 0) {
 	if (test->protocol->id == Pudp)
 	    blksize = DEFAULT_UDP_BLKSIZE;
+	else if (test->protocol->id == Psctp)
+	    blksize = DEFAULT_SCTP_BLKSIZE;
 	else
 	    blksize = DEFAULT_TCP_BLKSIZE;
     }

src/iperf_api.h

@@ -23,6 +23,7 @@ struct iperf_stream;
 #define Psctp 12
 #define DEFAULT_UDP_BLKSIZE 8192
 #define DEFAULT_TCP_BLKSIZE (128 * 1024)  /* default read/write block size */
+#define DEFAULT_SCTP_BLKSIZE (64 * 1024)
 
 /* short option equivalents, used to support options that only have long form */
 #define OPT_SCTP 1