ofp-util: Add functions to support version number bitmaps.

The bitmap is simple a uint32_t, allowing for versions up until
wire-protocol number 31.

Bit offsets correspond to ofp_version numbers which in turn
correspond to wire-protocol numbers for Open Flow versions..
E.g. (1 << OFP10_VERSION) is the mask for Open Flow 1.1.
If the bit for a version is set then it is allowed, otherwise it is
disallowed.

This is in preparation for allowing the range of allowed OpenFlow
versions to be configured.

Signed-off-by: Simon Horman <horms@verge.net.au>
[blp@nicira.com simplified some functions to constants, and rewrote
 a few functions in terms of utility bitwise functions]
Signed-off-by: Ben Pfaff <blp@nicira.com>