Since we're no longer always returning a getenv result or some defined
string, the callers should cleanup the variable after use.

As a result, change from const char* to char*, add the needed free()
everywhere and use strdup() on strings coming from getenv.
Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>