minor code cleanup

The attached patch makes the following cosmetic improvements:

- replace some function signatures of the form "some_type foo()" with
"some_type foo(void)"
- replace a few instances of a literal 0 being used as a NULL pointer;
there are more instances of this in the code, but I just fixed a few
- in src/backend/utils/mb/wstrncmp.c, replace K&R style function
declarations with ANSI style, remove use of 'register' keyword
- remove an "extern" modifier that was applied to a function definition
(rather than a declaration)

(These changes were made initially to satisfy "sparse", but IMHO they
are all good style anyway.)

Barring any objections, I intend to apply this patch on Sunday or Monday.

-Neil