Fix ALTER OPERATOR to update dependencies properly.

Fix an oversight in commit 321eed5f: replacing an operator's
selectivity functions needs to result in a corresponding update in
pg_depend.  We have a function that can handle that, but it was not
called by AlterOperator().

To fix this without enlarging pg_operator.h's #include list beyond
what clients can safely include, split off the function definitions
into a new file pg_operator_fn.h, similarly to what we've done for
some other catalog header files.  It's not entirely clear whether
any client-side code needs to include pg_operator.h, but it seems
prudent to assume that there is some such code somewhere.