Enforce consistent limits of daemons run from rc.subr:

• Allow the user to configure the login class to use in rc.conf by using {daemon}_login_class, which;
• Use the daemon class by default;
• .. and then use 'limits' to set the login class so it works both via init at startup (which runs this in 'daemon' class) and via whichever root environment (eg command line, other daemons, etc.)

Reviewed by: dteske
Differential Revision: https://reviews.freebsd.org/D3630