BUG/MINOR: tools: use my_unsetenv instead of unsetenv

BUG/MINOR: tools: use my_unsetenv instead of unsetenv

Let's use our own implementation of unsetenv() instead of the one, which is
provided in libc. Implementation from libc may vary in dependency of UNIX
distro. Implemenation from libc.so.1 ported on Illumos (see the link below) has
caused an eternal loop in the clean_env(), where we invoke unsetenv().

(https://github.com/illumos/illumos-gate/blob/master/usr/src/lib/libc/port/gen/getenv.c#L411C1-L456C1)

This is reported at GitHUB #3018 and the reporter has proposed the patch, which
we really appreciate! But looking at his fix and to the implementations of
unsetenv() in FreeBSD libc and in Linux glibc 2.31, it seems, that the algorithm
of clean_env() will perform better with our my_unsetenv() implementation.

This should be backported in versions 3.1 and 3.2.