man: add mention that libsystemd uses getenv()

See #26688: getenv() is not thread-safe, and could a possible source of
problems when a multi-threaded program calls setenv()/putenv()/unsetenv() in
parallel. It is not possible to avoid getenv() calls in general, since $PATH,
$LANG, $SHELL, $USER, $HOME, $TZ may need to be accessed at any time.
Add a warning to our docs so that people are aware of the issue.

Closes #26688. (Real fixes will need to be in glibc and gnome-shell or other
programs.)

The text is added to threads-aware.xml to be included in various places. By
including it in libsystemd-pkgconfig.xml, it is automatically added to all sd-*
pages. The text is also included explicitly in pages for a few other functions
which are call getenv().

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