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

[kib]

Summary

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.

[brooks]

Seems fine overall.

fdopendir.c almost certainly shouldn't have a Regents copyright. @delphij wrote the initial trivial version in 6fda52ba75654d8a540536228390922bfae1db69.