gpg: Simplify keydb handling of the main import function.

* g10/import.c (import_keys_internal): Return gpg_error_t instead of
int.  Change var names.
(import_keys_es_stream): Ditto.
(import_one): Ditto.  Use a single keydb_new and simplify the use of
of keydb_release.

Note that this opens a keydb handle before we call
get_pubkey_byfprint_fast which internally uses another key db handle.
A further patch will cleanup this double use. Note that we also
disable the keydb caching for the insert case.

The s/int/gpg_error_t/ has been done while checking the call chains of
the import functions and making sure that gpg_err_code is always used.

• Signed-off-by: Werner Koch <wk@gnupg.org>