ovsdb: Allow comparison on optional scalar types

This allows things like initiating a wait request on an interface
ofport being set.

When the optional field is empty and operation is != or excludes
then the result is true; otherwise it is false. If the field is
set then the field is compared normally for its type.

Signed-off-by: Terry Wilson <twilson@redhat.com>
[blp@nicira.com updated ovsdb-server(1) and NEWS.]
Signed-off-by: Ben Pfaff <blp@nicira.com>
tests: Add test that 1.5 copy-field can copy into the standardized xregs.

Signed-off-by: Jean Tourrilhes <jt@hpl.hp.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>