monitor: simplify abstract socket logic

monitor: simplify abstract socket logic

Older version of liblxc only allowed for 105 bytes to be used for the abstract
unix domain socket name because the code for our abstract unix socket handling
performed invalid checks. Since we \0-terminate we could now have a maximum of
106 chars. But do not break backwards compatibility we keep the limit at 105.

Reported-by: 0x0916 w@laoqinren.net
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>