Fix __warn_unused_result__, add more of it and fix the resulting warnings.

Fix __warn_unused_result__, add more of it and fix the resulting warnings.

Due to a bug in the autoconf test for function attributes, we were always
disabling __warn_unused_result__. Fix this, and add this function attribute
to a lot more functions whose results are definitely important.

This change makes it clear where we ignore the results of a function that
might fail. The proper fix in most cases is to propagate the result to the
caller. For meshlink_blacklist() and meshlink_whitelist(), we were not
return an error condition, even if we might fail to commit the blacklist
operation to permanent storage. So we now make these functions return a
bool.