cindex fails with mmap errors on OpenBSD

```
Trying to run codesearch on OpenBSD -current amd64 with go 1.0.3 from packages,
using current codesearch code fetched with 'go get', I'm seeing this:

$ go run src/code.google.com/p/codesearch/cmd/cindex/cindex.go /usr/src
2013/03/04 10:52:52 index /usr/src
2013/03/04 10:56:11 flush index
2013/03/04 10:56:12 merge 13 files + mem
2013/03/04 10:56:12 mmap /tmp/csearch-index060279584: errno 4967816
exit status 1

If I run tests I get this,

=== RUN TestMerge
2013/03/04 11:11:32 merge 0 files + mem
2013/03/04 11:11:32 99 data bytes, 1217 index bytes
2013/03/04 11:11:33 merge 0 files + mem
2013/03/04 11:11:33 87 data bytes, 1217 index bytes
2013/03/04 11:11:33 mmap /tmp/index-test259895843: errno 5172144
exit status 1
FAIL code.google.com/p/codesearch/index 0.411s

The errno seems strange, any suggestions what might be happening? OpenBSD does
not have UBC so I'm wondering if it may require msync (not sure if cindex is
mixing access via standard file operations and mmap, but if so, it will need
msyncs between the different types of access).
```

Original issue reported on code.google.com by `s...@spacehopper.org` on 4 Mar 2013 at 11:19

Inizia riproducendo il malfunzionamento su OpenBSD con cmd/cindex/cindex.go e il TestMerge del package index, usando gli errori mmap segnalati come sintomo da investigare. Esamina come cindex e TestMerge creano, scrivono e mappano i loro file di indice temporanei. Il lavoro è completato quando il percorso di indicizzazione segnalato e TestMerge non falliscono più con errori mmap su OpenBSD.

Scritto dal modello di indicizzazione a partire dal testo della issue.