Support zero-copy send in fio_sec_osd to allow testing it

Prelimilary results:
- CPU usage drops significantly. For example, in T1Q8 128K write test against
  stub_uring_osd with 10G network and Athlon X4 860k CPU it drops from 100% to 30%
- Latency becomes slightly worse. In T1Q1 4K write test in the same environment
  latency increases from 56 to 63 us.
- Small write throughput also becomes slightly worse. In T1Q128 4K write test
  against stub iops decreases from 138k to ~110k (unstable, fluctuates 100k..120k).
  Note that this is without io_uring, of course.

src/rw_blocking.h

@@ -10,3 +10,4 @@ int read_blocking(int fd, void *read_buf, size_t remaining);
 int write_blocking(int fd, void *write_buf, size_t remaining);
 int readv_blocking(int fd, iovec *iov, int iovcnt);
 int writev_blocking(int fd, iovec *iov, int iovcnt);
+int sendv_blocking(int fd, iovec *iov, int iovcnt, int flags);