Ticket #3025: race condition when creating temporary directory.

When there's no mc-tmpdir and a user tries to start two mc sessions
simultaneously, sometimes (in one out of ten attempts on my machine) one
mc session emits the following error message:

Cannot create temporary directory /tmp/mc-lars: File exists (17)
Temporary files will be created in /tmp
Press any key to continue...

Steps to reproduce:

# rm /tmp/mc-$(whoami) -rf
# uxterm -e mc & uxterm -e mc

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Merge branch '3025_race_cond_reate_tmp_dir'

* 3025_race_cond_reate_tmp_dir:
  Ticket #3025: race condition when creating temporary directory.