libc: Add getenv_r() function.

[des]

Summary

This is a calque of the NetBSD function of the same name.

MFC after: never
Relontes: yes
Sponsored by: Klara, Inc.

[kevans]

If you're up for it, adding _FORTIFY_SOURCE support for this would be nearly trivial. The __ssp_redirect pattern for arc4random_buf in include/ssp/stdlib.h can be copied for this (the __buf and __len names are significant and used in the implementation for size-checking, other names are fair game) and tests added easily by copying arc4random_buf again in lib/libc/tests/secure/generate-fortify-tests.lua (regeneration requires devel/lua-luacheck for make generate-tests).

[markj]

lib/libc/stdlib/getenv.c

475

Wouldn't it be more useful to somehow return the actual size of the variable? Userspace effectively has to guess and just double the size until getenv_r() succeeds.

[imp]

lib/libc/stdlib/getenv.c

475

Worse, we copy to the buffer and return an error....