Fix strict aliasing violations with GCC 4.1 and 4.4.

Fix strict aliasing violations with GCC 4.1 and 4.4.

The typical use of struct sockaddr_storage is flagged as a strict
aliasing violation by GCC 4.4.7.  Using an explicit union lets the
compiler know that accessing the same location via different types is
not an error.

GCC 4.1.2 had a similar complaint about a cast of ukey's key_buf to
nlattr.  After this patch there are no further warnings with the
XenServer build, so we could start treating warnings as errors in the
builds.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>