nss-imap: add sss_nss_getsidbyuid() and sss_nss_getsidbygid()
Two new calls are added to allow the caller to specify if the given
POSIX ID is a UID or a GID and the expected result is a user or a group
respectively. This is needed because on POSIX a user and a group may
share numerically the same ID value but might have different SIDs
assigned.
Related to https://pagure.io/SSSD/sssd/issue/3629
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>