indexfile: use mmap on darwin

We can use mmap on darwin. I additionally renamed indexfile files to match the
usual names used when differentiating between different build tags.

Change-Id: Ie712ee0e7e3634a9bf4e0cb4fd9a0670c4846fc3

indexfile_other.go

diff --git a/indexfile.go b/indexfile_other.go
similarity index 97%
rename from indexfile.go
rename to indexfile_other.go
index a6fc12e..1c03fe8 100644
--- a/indexfile.go
+++ b/indexfile_other.go
@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-// +build !linux
+// +build !linux,!darwin
 
 package zoekt
 

indexfile_unix.go

diff --git a/indexfile_linux.go b/indexfile_unix.go
similarity index 98%
rename from indexfile_linux.go
rename to indexfile_unix.go
index 14655ae..d7e9402 100644
--- a/indexfile_linux.go
+++ b/indexfile_unix.go
@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// +build linux darwin
+
 package zoekt
 
 import (