WL#1580: --start-datetime, --stop-datetime, --start-position (alias for --position) and --stop-position
options for mysqlbinlog, with a test file.
This enables user to say "recover my database to how it was this morning at 10:30"
(mysqlbinlog "--stop-datetime=2003-07-29 10:30:00").
Using time functions into client/ made me move them out of sql/ into sql-common/.
+ (small) fix for BUG#4507 "mysqlbinlog --read-from-remote-server sometimes
cannot accept 2 binlogs" (that is, on command line).