ofpbuf: New function ofpbuf_const_initializer().

ofpbuf: New function ofpbuf_const_initializer().

A number of times I've looked at code and thought that it would be easier
to understand if I could write an initializer instead of
ofpbuf_use_const().  This commit adds a function for that purpose and
adapts a lot of code to use it, in the places where I thought it made
the code better.

In theory this could improve code generation since the new function can
be inlined whereas ofpbuf_use_const() isn't.  But I guess that's probably
insignificant; the intent of this change is code readability.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Jarno Rajahalme <jarno@ovn.org>