dirent-util: introduce simple wrapper of posix_getdents()

dirent-util: introduce simple wrapper of posix_getdents()

glibc exports getdents64 syscall as is, but musl exports it as
posix_getdents(). Let's introduce a simple wrapper of posix_getdents().

Note, our baseline for glibc is 2.31. Hence, we can assume getdents64()
always defined when building with glibc.