The configuration reader/writer uses mmap(2) interface to access
the files; when we find a directory, it barfed with "Out of memory?".

* jk/diagnose-config-mmap-failure:
  xmmap(): drop "Out of memory?"
  config.c: rewrite ENODEV into EISDIR when mmap fails
  config.c: avoid xmmap error messages
  config.c: fix mmap leak when writing config
  read-cache.c: drop PROT_WRITE from mmap of index