[libpng18] chore: Clean up the FILE * formulations in code and in documentation

We should use `FILE *` instead of `FILE*` or `(FILE*)`, consistently,
as we should for all other pointer types. Moreover, when we refer to
standard stdio file objects in comments and in documentation, we should
use the term "FILE objects" consistently.

Lastly, we clarify in a comment in example.c that `PNG_STDIO_SUPPORTED`
is true only when the stdio support is both available in the system and
accessible in the user's libpng build.

This is a cherry-pick of commit c63c5463903014c904b540216c2784023fb8c1c8
from branch 'libpng18'.

Reviewed-by: John Bowler <jbowler@acm.org>
Signed-off-by: Cosmin Truta <ctruta@gmail.com>

contrib/examples/pngpixel.c

@@ -184,11 +184,11 @@ int main(int argc, const char **argv)
                      compression_method, filter_method;
                   png_bytep row_tmp;
 
-                  /* Now associate the recently opened (FILE*) with the default
-                   * libpng initialization functions.  Sometimes libpng is
-                   * compiled without stdio support (it can be difficult to do
-                   * in some environments); in that case you will have to write
-                   * your own read callback to read data from the (FILE*).
+                  /* Now associate the recently opened FILE object with the
+                   * default libpng initialization functions.  Sometimes libpng
+                   * is compiled without stdio support (it can be difficult to
+                   * do in some environments); in that case you will have to
+                   * write your own read callback to read data from the stream.
                    */
                   png_init_io(png_ptr, f);