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
