core: Allow logging to an estream.
* src/logging.c (set_file_fd): Add and use new arg 'stream'. (_gpgrt_log_set_sink): Implement setting an estream sink. * tests/t-logging.c: New test. * tests/Makefile.am (TESTS): Add test.
- Signed-off-by: Werner Koch <wk@gnupg.org>