libc: Change dirfd() into a function (needed per POSIX).

At the same time, make struct _dirdesc private to libc. If access
to dd_fd is needed, dirfd() has to be used. Inside libc, the former
dirfd()

libc: Change dirfd() into a function (needed per POSIX).

At the same time, make struct _dirdesc private to libc. If access
to dd_fd is needed, dirfd() has to be used. Inside libc, the former
dirfd() macro is preserved as _dirfd().

Also, in struct _dirdesc, change the lock from "void *" to
"struct pthread_mutex *". This allows us to eliminate some casts
in readdir() and friends.

Our documentation already describes dirfd() as a function.

Based-on: FreeBSD's similar work

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