Avoid leaving temporary batch files in the temporary directory.

Avoid leaving temporary batch files in the temporary directory.

 job.c (create_batch_file): Declare the counter of batch files
 static, to avoid having 2 jobs using the same file name and
 stepping on each other's toes.  When all 64K names are used up,
 make one more loop looking for slots that became vacant.  This
 avoids leaving behind temporary batch files in the temporary
 directory, which happens frequently on a fast machine when using
 parallel builds.