Add config file handling

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Document configuration file

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Add keyring options

Add --keyring and --no-keyring to the command line options and keyring to
section [general] in the configuration file.

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Get rid of fill_config_with_default_values

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Check if selected backend is valid

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Enable keyring per default

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Merge branch 'config'

see pull request 84
add missing comma
remove print_error2() call
more detail in config man page

Show usage for boolean.
The user setting will probably be used by many users.
update AUTHORS

Not sure if I should include email addresses.
Ping me if you want yours shown or wouldn't like yours
shown here.
fix parsing of positional arguments again

We allow giving user/device directly as positional arguments,
without giving "--user" and "--device".
This was broken after the merge of the config file branch.
fix tests concerning options.user

The user can be set per config file
so it isn't always empty when not given as parameter.