Split out process check as separate function.

This will allow the checks to be changed for different operating systems.

Issue #260 is related to this, but this does not change any current behaviour.

lock_windows.go

@@ -1,10 +1,25 @@
 package restic
 
 import (
+	"os"
 	"os/user"
+
+	"github.com/restic/restic/debug"
 )
 
 // uidGidInt always returns 0 on Windows, since uid isn't numbers
 func uidGidInt(u user.User) (uid, gid uint32, err error) {
 	return 0, 0, nil
 }
+
+// checkProcess will check if the process retaining the lock exists.
+// Returns true if the process exists.
+func (l Lock) processExists() bool {
+	proc, err := os.FindProcess(l.PID)
+	if err != nil {
+		debug.Log("Lock.Stale", "error searching for process %d: %v\n", l.PID, err)
+		return false
+	}
+	proc.Release()
+	return true
+}