this allows to leave the password field empty in the
secrets file. the parser will not give an error if it
only finds two fields in a line in the secrets file.
later it will only ask for the password, silently
using the configured username. mount.davfs also takes
the password from <stdin>, therefore no need to
specify the password in a configuration file.
this also helps to get davfs2 working together with
pam_mount, there the password is passwd via stdin to
the mount command.