port "Report timing with more precision" by @dveeden

based upon: dveeden/mysql-server@d5e4642

MariaDB [test]> select sleep(0.123);

+--------------+

| sleep(0.123) |

+--------------+

| 0 |

+--------------+

1 row in set (0.123 sec)

"More exact timing for mysql client based on my_timer_microseconds"

Based on suggestion from @grooverdan on mysql/mysql-server#112

This patch is slightly bigger because the original did not preserve the

return type of my_timer_microseconds and this patch does.

(my_timer_microseconds returns ulonglong, not simply ulong)

Also I believe the correct place to do the division of microseconds to

seconds is better in the caller of nice_time because nice_time takes a

"double sec" param, so we should convert before calling; the other caller

of nice_time does not call with microseconds.