WL#1277: Allow specify slow_query_time in fractions of seconds

We shall allow to specify slow_query_time with better accuracy than seconds.
We shall also log query times in slow query log with better than second acuracy.

The speed of gettimeofday() seems to be pretty similar to time() at least on
Linux and Solaris so this shall not be performance problem. (See WL#2373.)

It is to be discussed if we shall allow to specify fractions (ie 0.5) or shall
use milliseconds instead (ie 500) for slow_query_time.

Georg Richter's changeset from 2005-08-16:
http://lists.mysql.com/internals/28330

Feature Requests:
BUG#6238 long_query_time less then 1
BUG#8043 long query time only integers
BUG#25412 Patch for microseconds in slow-log (from Vadim Tkachenko)

To keep things compatible with older releases, the argument of slow_query_time
was changed to be a decimal number instead of changing it to microseconds.

In other words, to specify a timing interval of 500 microseconds, one should use
--slow-query-time=0.5