I had the same issue on Gentoo: also boot stoped on line `nm-dispatcher: req:3 'hostname': start running ordered scripts...`
Yesterday resolved. I installed sysklogd:
rc-update add sysklogd default
After this all boot Ok, except NetworkManager - it does not start. I tried to find where it added to autoboot and didnot found.
So, i uncommened in /etc/conf.d/netmount :
And all works well!
As debug used links: https://sparkylinux.org/forum/index.php?topic=4018.0 - to find that lightdm can output errors to syslog.
Also: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=852469 - i have the same kernel (4.9).