add get_[static/dynamic/pretty]hostname functionality

dynamic hostname (sometimes just hostname) is always whatever
the gethostname(3) returns.

static hostname is derived from prettyhostname, which attempts to
convert it to a 7 bit ascii hostname (no bookend -'s, etc.) if this
fails or prettyhostname has not yet been set, defaults to gethostname(3).

prettyhostname is whatever is in /etc/systemd_compat.conf, if reading that
fails it is just an empty string. prettyhostname is blank until manually set.