unit-name: never create a unit name with a leading '.'

unit-name: never create a unit name with a leading '.'

Supposed to prevent creating unit files like:
  ├── dev-sda1.device.wants
  │   └── .dot.mount -> /run/systemd/generator/.dot.mount
  ├── .dot.mount
from:
  # cat /etc/fstab
  /dev/sda1    /.dot           vfat ro           1 3

which we later skip reading because of the leading '.'.