libc/csu: rename ignore_init.c to libc_start1.c

The current name was a historical curiosity that started when init array
support was added, and then the file appeared a convenient place for the
addition of the MI common code to csu. It is now referenced by name in
single place and the rename is easy, so do it.

Sponsored by: The FreeBSD Foundation
MFC after: 3 weeks