libc/gen: split user-visible opendir()-like functions into separate source files

This allows, for instance, to interpose opendir() with an implementation
that calls into fdopendir(), without causing multiple symbol definitions
when libc is linked in statically.

This is aligned with the quality of implementation principle that single
object file from a static library should not provide more than one
app-visible symbol.

[The copyright for fopendir() was copied, it might be refined by the
author]

Reviewed by: brooks
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D49089