Shared benchmark format for command line and CSV outputs

1. add `print_output_table_header_row` func to print the table header for benchmark output
2. modify the following benchmarks to include the table header
    - bench_ecdh.c
    - bench_ecmult.c
    - bench_internal.c
    - bench_recover.c
    - bench_schnorrsig.c
    - bench_sign.c
    - bench_verify.c

src/bench_internal.c

@@ -344,6 +344,7 @@ void bench_context_sign(void* arg, int iters) {
 int main(int argc, char **argv) {
     bench_inv data;
     int iters = get_iters(20000);
+    print_output_table_header_row();
 
     if (have_flag(argc, argv, "scalar") || have_flag(argc, argv, "add")) run_benchmark("scalar_add", bench_scalar_add, bench_setup, NULL, &data, 10, iters*100);
     if (have_flag(argc, argv, "scalar") || have_flag(argc, argv, "negate")) run_benchmark("scalar_negate", bench_scalar_negate, bench_setup, NULL, &data, 10, iters*100);