rotatelogs: Use apr_psprintf() with %pm instead of a constant length buffer for
errors.

* support/rotatelogs.c
  (post_rotate, doRotate): Switch to using apr_psprintf() with %pm.

Suggested by: rpluem


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1534895 13f79535-47bb-0310-9956-ffa450edef68