gpg: Add new option --debug-iolbf.

* g10/gpg.c (oDebugIOLBF): new.
(opts): Add --debug-iolbf.
(main): Set option.

This option is convenient for debugging to make sure that debug output
to stderr is synced with output to stdout.

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