[cmaster-next] Snapcraft_v2 branch ready for merge into stable/2.0

[Martin Winter]

Got all the required changes from Renato and have now a branch
with all the Snapcraft parts ready for merge.
This includes code to modify the main Quagga and (in the snapcraft
subdir) all the needed files to build a snap.

Doc / Package files will need one more round to adjust mainly for the
name (once we settle on something)

Branch is  snapcraft_v2

Main changes:

- Snap packages are only allowed to write into their own mounted 
container and the
   filenames are not known until the package is installed. There are now 
new —vty_socket
   cli options to specify the location for the vty socket instead of 
using the compile-time
   path. (Plus —ctl_socket for the extra LDP socket and —config_dir 
for vtysh)

- Snap packages can’t even read files outside their directories. 
Getting the homedir
   from the password file isn’t possible. Using now HOME env variable 
and only fall back
   to passed file if it doesn’t exits

- Snap packages can’t SETUID or SETGID. They always run under root. 
There is now a check
   for UID and GID and the change only happens if it’s not already 
running under the
   requested User/Group

- Martin