gpg-interface: fix misdesigned signing key interfaces

gpg-interface: fix misdesigned signing key interfaces

The interfaces to retrieve signing keys and their IDs are misdesigned as
they return string constants even though they indeed allocate memory,
which leads to memory leaks. Refactor the code to instead always return
allocated strings and let the callers free them accordingly.

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