rename error.log to nginx-error.log for clarity
Fix some cleanup issues

 - php-cgi processes weren't killed when dash is the shell (eg Ubuntu 14.04)
   This is because proc_open() + proc_status() gets you the process of the
   shell instead of the executable. Same behavior with PHP5.
 - if Siege is ctrl-c'ed early, we could try to unlink an already-removed file