If no trustlist.txt file is found, gpg-agent sends debug output to the assuan
status fd ("gpg-agent[5290]: error reading list of trusted root
certificates\ngpg-agent[5290]: command is_trusted failed: No such file or
directory\nERR 67141713 No such file or directory <GPG Agent>\n"

This causes gpgsm to be confused in assuan_transact and issue a
GPG_ERR_ASS_INV_RESPONSE error.

Three bugs here: (1) gpg-agent should not send debug output to assuan fd. (2)
gpg-agent should fail gracefully if trustlist.txt is not available and assume
"not-trusted" (argueably; otherwise gpgsm should install empty trustlist file).