Write pgplot png files directly

Do not make "pgplot.png" and rename.  While pgplot.png is probably
created on the same filesystem as the intended plot, that is not
guaranteed, so 'rename' may be unsafe.  Also, it is perhaps unwise to
have the three plotting routines sharing the same-named temporary
file; this change eliminates the use of the temporary file altogether.
Move AGASC tasks out of eval plot block

Starcheck needs the stars to check for spoilers and such even if in
no-plot mode (which never happens anyway).
Add a checks to confirm non-zero-size plots

Let each plotting routine check that it has successfully created a
plot file and that file has non-zero-size.  When pgplot fails to open
or write a file, it writes to STDERR but does not die or
return good status info (pgbegin seems to always return 1).  Without
explicit file checks, there could be a failure to create the plots
without a corresponding warning() in STDERR and in the processing
warnings in the starcheck output.  This changes adds those checks.
Wait and retry to make plots on plot errors

If any of the plot creation routines errored out or did not
successfully create a non-zero png file, wait and retry all three plot
routines.  The hope is that a pause of a few seconds will be enough
time to get over an NFS hiccup.
Merge pull request #49 from sot/pgplot_cleanup

Make plot creation more robust