Popen: Allow fclose() to close popen FILE handle

The user may do:
  fp = popen(...); fclose(fp);
Glibc doesn't mind this, thus we shouldn't eithre.

git-svn-id: svn://svn.code.sf.net/p/dmtcp/code/trunk@2816 ba6dc0ae-232f-0410-883f-9ebac64fc565