* src/context.h (struct gpgme_context): New field key_origin.
* src/engine-backend.h (struct engine_ops): Add arg key_origin to
field 'import'.
* src/engine-gpg.c (gpg_import): Add arg key_origin and pass option
--key-origin with argument value to gpg. Adjust all callers.
* src/engine-gpgsm.c (gpgsm_import): Add dummy arg key_origin.
* src/gpgme.c (gpgme_release): Free 'key_origin'.
(gpgme_set_ctx_flag, gpgme_get_ctx_flag): New flag "key-origin".
* tests/run-import.c (main): Add option --key-origin.
* tests/gpg/t-import.c (main): Set and verify key origin.