Trace the use of GPG_ERR_INV_ENGINE.

* src/debug.h: Include "gpgme.h"
(_gpgme_trace_gpgme_error): New.
(trace_gpg_error): New macro.  Use it in all files where we return
GPG_ERR_INV_ENGINE; also "include debug.h" as needed.

This is a pretty common error code but often it is hard to figure out
the actual cause. With debug level 4 we now print the file name and
line number where this error code is generated by gpgme. Along with
the git revision printed in the first log lines, this should give us
an easier way to track down the problems related to this error code.