core: Add error handling to setexpire

* src/genkey.c (setexpire, gpgme_op_setexpire_start,
gpgme_op_setexpire): Move to ...
* src/setexpire.c: New.
* src/Makefile.am (main_sources): Add that file.
* src/context.h (ctx_op_data_id_t): Add OPDATA_SETEXPIRE.
* lang/qt/tests/t-various.cpp (testSetExpire): Test error handling.

Errors (and failures) emitted via status-fd need to be handled
explicitly, i.e. we need to provide an appropriate status handler with
corresponding op_data_t. Additionally, we need to set a passphrase
command handler if a passphrase callback is set in the context, e.g.
during tests.

• GnuPG-bug-id: T4395