Add MMAP file pattern to apcu conf

Part of https://github.com/cargomedia/puppet-cargomedia/issues/585

We experienced php segfaulting (`SIGSEGV`) on both php5-fpm and CLI.
This happened specifically when there was *not too much* memory available (e.g. 70% used).

Turns out not using *anonymous* mmapping helps... (setting `apc.mmap_file_mask`).