Fix : gp_gettmpdir was inconsistent to gp_open_scratch_file.

This is a preliminary publication.
The patch was not tested on some platforms.


[Log message beg]
Fix : gp_gettmpdir was inconsistent to gp_open_scratch_file.

DETAILS :

The old code wrongly implemented gp_gettmpdir, returning a result
inconsistent with one assumed by gp_open_scratch_file.
Particularly on Unix when TMPDIR, TEMP environment variables are not set,
it retrieved an empty string while gp_open_scratch_file places files to
/tmp/ .
This caused a security hole.

The new code provides a platform dependent code for gp_gettmpdir,
and splits gp_open_scratch_file into 2 functions :
gp_open_scratch_file compatible to the old implementation except for
ztempfile,
and gp_open_scratch_file_generic, which allows clients to tell whether the
prefix is bare
(i.e. starts neither from root nor from cwd nor from parent).

Now the logics of ztempfile is straight-forward :

- obtain a path for scratch file;
- check it for permissions, unless it is in a temp directory;
- open a scratch file on the obtained path.
[Log message end]