Improvements to sock_diag code.

1. Add a Dump convenience function that takes a request.
2. Add code to zero out uninitialized data in diag_msg.
3. Add more granular functions for constructing requests
   (e.g., from sockets) without executing dumps.
4. Rename the Get* functions that scan socket dumps (such as
   GetSockDiagForFd) to Find* to make it clear that they scan
   dumps instead of asking the kernel for a particular socket.

Change-Id: I62ea60f888fe2dd50f5c5cf9dd680608a74097a3