Unbreak several notmuch commands after the addition of configuration.

Unbreak several notmuch commands after the addition of configuration.

All of the following commands:

	notmuch dump
	notmuch reply
	notmuch restore
	notmuch search
	notmuch show
	notmuch tag

were calling notmuch_database_open with an argument of NULL. This was
a legitimate call until the recent addition of configuration, after
which it is expected that all commands will lookup the correct path in
the configuration file. So fix all these commands to do that.

Also, while touching all of these commands, we fix them to use the
talloc context that is passed in rather than creating a local talloc
context. We also switch from using goto for return values, to doing
direct returns as soon as an error is detected, (which can be leak
free thanks to talloc).