MySQL: determine slave-ness before stopping

This makes it easier to perform later logic (to use SLAVE STATUS or
MASTER STATUS). Also
Mysql: use correct log file and position

According to several experts (including Percona), when issuing "CHANGE
MASTER TO" you are specifying the position in the master binary log that
the slave will start executing from.

Exec_Master_Log_Pos and Relay_Master_Log_File are the correct positions
to save when reloading this snapshot and issuing a "CHANGE MASTER TO"
command.

Percona:
http://www.mysqlperformanceblog.com/2008/07/07/how-show-slave-status-relates-to-change-master-to/

Pythian:
http://www.pythian.com/blog/beware-starting-slaves-in-position-in-the-master-info-file/
Merge (#1) mysql--stop-slave-option into ifixit-production

Mysql: add option to issue "STOP SLAVE" command
instead of "FLUSH TABLES WITH READ LOCK".

mysql has an issue with read locks and it's very easy to effectively
lock up your whole DB with a SELECT. If a mysql slave is being
snapshotted, "STOP SLAVE" is enough to ensure no more writes happen and
is guaranteed not to lock the DB.

See: http://www.mysqlperformanceblog.com/2012/03/23/how-flush-tables-with-read-lock-works-with-innodb-tables/

Also, the log file and positions were being captured incorrectly.