mallinfo-util: assume mallinfo() exists

mallinfo-util: assume mallinfo() exists

The check existed for musl. Let's remove it, as we explicitly request glibc.

While removing the check, this also drops generic_mallinfo, introduces
a tiny converter from struct mallinfo to struct mallinfo2 if mallinfo2()
does not exist, and renames mallinfo-util.h to malloc.h.

With this change, we can drop many ifdefs and casts in .c files.