core: Prettify _gpgme_io_select debug output again and fix TRACE_SYSRES.

* src/debug.c (_gpgme_debug): Take better care of NULL userinfo.
(_gpgme_debug_end): Rework.
(_trace_sysres): Print ERRNO and not the supplied RES.

The TRACE_SYSRES patch fixes

• Regression-due-to: 7a1e7006d06fdbab86ea79a197c316744b09d933
• Signed-off-by: Werner Koch <wk@gnupg.org>