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>

environment.h

@@ -0,0 +1,12 @@
+#ifndef ENVIRONMENT_H
+#define ENVIRONMENT_H
+
+#include "strvec.h"
+
+/*
+ * Wrapper of getenv() that returns a strdup value. This value is kept
+ * in argv to be freed later.
+ */
+const char *getenv_safe(struct strvec *argv, const char *name);
+
+#endif