util: New macro CONST_CAST.

util: New macro CONST_CAST.

Casts are sometimes necessary.  One common reason that they are necessary
is for discarding a "const" qualifier.  However, this can impede
maintenance: if the type of the expression being cast changes, then the
presence of the cast can hide a necessary change in the code that does the
cast.  Using CONST_CAST, instead of a bare cast, makes these changes
visible.

Inspired by my own work elsewhere:
http://git.savannah.gnu.org/cgit/pspp.git/tree/src/libpspp/cast.h#n80

Signed-off-by: Ben Pfaff <blp@nicira.com>