environment: make `getenv_safe()` a public function

environment: make `getenv_safe()` a public function

The `getenv_safe()` helper function helps to safely retrieve multiple
environment values without the need to depend on platform-specific
behaviour for the return value's lifetime. We'll make use of this
function in a following patch, so let's make it available by making it
non-static and adding a declaration.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>