Convert function declarations from K&R to ANSI style.

cproto (http://invisible-island.net/cproto/) was used to do the bulk of
the work in batch; even the most recent version had some issues with
typedef'd parameters; a quick "patch" was to modify `lex.l` to
explicitly include all vim typedefs as known types. One example from
`vim.h` is

    typedef unsigned char char_u;

which was added in `lex.l` as

    <INITIAL>char_u    { save_text_offset(); return T_CHAR; }

Even with these changes there were some problems:

* Two files (`mbyte.c` and `os_unix.c`) were not cleanly converted.
* Any function with the `UNUSED` macro in its parameter list was not converted.

Rather than spend more time fixing the automated approach, the two files
`mbyte.c` and `os_unix.c` were converted by hand.

The `UNUSED` macros were compiler specific, and the alternative, generic
version would require a different syntax, so in order to simplify the
conversion all uses of `UNUSED` were stripped, and then the sources were
run back through cproto. It is planned to reconsider each use of
`UNUSED` manually using a new macro definition.
README.md: cosmetic fixes