WHERE INNODB WRITES TEMP FILES

Problem:
========
InnoDB creates temporary files for online ALTER statements in the tmpdir.
In some cases, the tmpdir is too small, or for other reasons, not the best
choice.

Solution:
=========
Create a new dynamic session variable "innodb_tmpdir"
that would determine where the temp files should create during alter
operation.

Behaviour of innodb_tmpdir :
===========================
1) Default value is NULL.
2) Valid inputs are any path other than mysql data directory path.
3) Directory Permission and existence checked as a part of validation for
   innodb_tmpdir.
4) If value is set to NULL, then temporary file created in the location of
   mysql server variable(--tmpdir).
5) User should have file privilege to set the variable.
6) If user provides a path which is symlink, then we resolve and store
   absolute path in innodb_tmpdir.
7) Path should not exceed 512 bytes.
8) Path should be a directory.

Reviewed-by: Marko <Mäkelä&lt;marko.makela@oracle.com>
Reviewed-by: Harin <Vadodaria&lt;harin.vadodaria@oracle.com>
Reviewed-by: Jon Olav <Hauglid&lt;jon.hauglid@oracle.com>
RB: 7628