fix race conditions reading/writing datafile

[meonkeys]

Description and repro:

If several processes simultaneously try to read and write the datafile, you may wind up with an empty datafile.

I noticed this when opening up many shells at once with screen -c dev, where dev contains:

source /etc/screenrc
screen -t home
chdir /tmp
screen -t tmp1
screen -t tmp2
screen -t tmp3
screen -t tmp4
screen -t tmp5

In one or more of the screen windows I see

mv: cannot stat `/home/adamm/.j.tmp': No such file or directory

and I notice my datafile has been truncated.