- More common filename accross the PHP repository
- Additionally, this patch replaces some legacy form feed (FF or ^L)
  characters (for printers) to LF (\n) newline character.

The `<assert.h>` header file is part of the standard C89 headers [1] and
on older systems there needed to be also a manual check if header is
present.

Since PHP requires at least C89 manual check and the `HAVE_ASSERT_H`
symbol defined by Autoconf in configure.ac can be both removed [2].

This patch also removes unused <assert.h> includes where c files don't
use the `assert()` macro.

Refs:
[1] https://port70.net/~nsz/c/c89/c89-draft.html#4.2
[2] https://git.savannah.gnu.org/cgit/autoconf.git/tree/lib/autoconf/headers.m4

The `n_next` member of struct `bc_struct` is unused; the only code that
would use this member has been disabled since libbcmath has been
bundled nearly 17 years ago.  Apparently, `n_next` has been designed to
build a linked list of free numbers, but it is doubtful whether that
would be an improvement over relying on the efficiency of the ZendMM,
so we remove the remaining support altogether.

The ABI break probably affects nobody, but would be okay for a new
minor version according to our release process, anyway.

(cherry picked from commit 40e7baab3c90001beee4c8f0ed0ef79ad18ee0d6)

# consistency..