Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

mdoc: sort prologue macros.

Remove superfluous paragraph macro.

* Deprecate `elf_getshnum()`, `elf_getphnum()` and `elf_getshstrndx()` in
favour of `elf_getshdrnum()`, `elf_getphdrnum()` and `elf_getshdrstrndx()`
respectively.
* Add new manual pages for `elf_getshdrstrndx()`, `elf_getphdrnum()` and
`elf_getshdrnum()`.
* Add a deprecation warning for `elf_getshstrndx()`, `elf_getphnum()` and
`elf_getshnum()`.

Obtained from: elftoolchain
MFC after: 1 month