* src/context.h (struct gpgme_context): New field export_filter.
* src/engine-backend.h (struct engine_ops): Add arg export_filter to
field 'export' and field 'export_ext'.
* src/engine-gpg.c (export_common): Add arg export_filter and pass
option --export-filter with argument value to gpg.
(gpg_export, gpg_export_ext): Add arg export_filter and pass it to
export_common. Adjust all callers of the two functions.
* src/engine-gpgsm.c (gpgsm_export, gpgsm_export_ext): Add dummy arg
export_filter.
* src/gpgme.c (gpgme_release): Free 'export_filter'.
(gpgme_set_ctx_flag, gpgme_get_ctx_flag): New flag "export-filter".
* tests/run-export.c (main): Add option --export-filter.