Define and use the ARRAY_SIZE macro

A similar macro is defined in the Linux kernel [1].

To refactor the code I used a slightly modified Coccinelle script I found in
[2].

```diff
// Use the macro ARRAY_SIZE when possible
//
// Confidence: High
// Copyright: (C) Gilles Muller, Julia Lawall, EMN, DIKU.  GPLv2.
// URL: http://www.emn.fr/x-info/coccinelle/rules/array.html
// Options: -I ... -all_includes can give more complete results

@@
type T;
T[] E;
@@

- (sizeof(E)/sizeof(*E))
+ ARRAY_SIZE(E)

@@
type T;
T[] E;
@@

- (sizeof(E)/sizeof(E[...]))
+ ARRAY_SIZE(E)

@@
type T;
T[] E;
@@

- (sizeof(E)/sizeof(T))
+ ARRAY_SIZE(E)

@n@
identifier AS,E;
@@

- #define AS(E) ARRAY_SIZE(E)

@@
expression E;
identifier n.AS;
@@

- AS(E)
+ ARRAY_SIZE(E)
```

`spatch --in-place --sp-file array_size.cocci -I src/ -I build/include/ -I build/src/nvim/auto/ src/nvim/*.c`

[1] http://lxr.free-electrons.com/source/include/linux/kernel.h#L54
[2] http://www.emn.fr/z-info/coccinelle/rules/#macros

src/nvim/version.c

@@ -986,7 +986,7 @@ void intro_message(int colon)
   };
 
   // blanklines = screen height - # message lines
-  blanklines = (int)Rows - ((sizeof(lines) / sizeof(char *)) - 1);
+  blanklines = (int)Rows - (ARRAY_SIZE(lines) - 1);
 
   // Don't overwrite a statusline.  Depends on 'cmdheight'.
   if (p_ls > 1) {
@@ -1006,7 +1006,7 @@ void intro_message(int colon)
   row = blanklines / 2;
 
   if (((row >= 2) && (Columns >= 50)) || colon) {
-    for (i = 0; i < (int)(sizeof(lines) / sizeof(char *)); ++i) {
+    for (i = 0; i < (int)ARRAY_SIZE(lines); ++i) {
       p = lines[i];
 
       if (sponsor != 0) {