WL#1243: Allow separate temporary directory for slave thread

As normal MySQL tmp directory may be located on tmpfs file directory, one may
wish to have slave thread tmp directory in different location so slave
(=replicated) temporary tables will not be lost on server restart. 
Note that even after this task is done, slave temporary tables will still be
lost after mysqld's restart (because at restart we don't know the table's name).
This will be fixed later.
For now, having a separate disk-based tmpdir for the slave thread instead of the
tmpfs normal tmpdir guarantees that the tmp files created by the replication of
LOAD DATA INFILE are not lost when the machine reboots. And it's also a progress
to not lose the temp tables.
We already have --slave-load-tmpdir, so the solution is:
* rename --slave-load-tmpdir to --slave-tmpdir (keep the old name for compatibility)
* use --slave-tmpdir in place of --tmpdir everywhere in the slave thread
* update the manual.