bug#70764: 29.1; M-x gdb: don't ask me about debuginfod server if debuginfod is not in context

[Rolf Ade <rolf@pointsman.de>]

With 29.1 as well as with trunk(both tested with -Q) if I call M-x gdb I
get asked:

"Enable querying debuginfod servers for this session? (y or n)"

Even if I answer with "n" I get the message:

"No symbol \"debuginfod\" in current context."

This can't be right. Declining to use a feature should not be answered
with a message that it would not be available anyways (what the message
seem to tell me).

Digging around a bit it turns out there is a variable to customise the
behaviour here: gdb-debuginfod-enable-setting. Its default value is
'ask.

From a user perspective it only make sense to be ask about wanting
something if this something is avaliable.

If it is about the advertising aspect then why not more clear don't ask
questions and instead write a message like: "You cannot use debuginfod
servers. Consider using this marvelous feature".

rolf