MFC r235647 for newsyslog (by gleb):
Hide DIR definition by making it an opaque struct typedef.

Introduce dirfd() libc exported symbol replacing macro with same name,
preserve _dirfd() macro for internal use.

Replace dirp->dd_fd with dirfd() call. Avoid using dirfd as variable
name to prevent shadowing global symbol.

MFC r250545:
Some filesystems (NFS in particular) do not fill out the d_type field when
returning directory entries through readdir(3). In this case we need to
obtain the file type ourselves; otherwise newsyslog -t will not be able to
find archived log files and will fail to both delete old log files and to
do interval-based rotations properly.

MFC r251240:
We want to stat the archived log file rather than the logfile itself.
MFC r250941:
Include descriptions of the flags and fields printed when dumping the IPv6
prefix list.
MFC r250939:
Correct the description of the -a option: rtsol(d) -a will in fact send out
router solicitations on multiple interfaces.

MFC r251059:
- Fix wording.
- Improve the descriptions in the FILES section.