unix socket moved into /var/run instead of /tmp

On latest systems, httpd has virtual /tmp by default. That causes
mod_netconf.sock cannot be found.
unix socket: changed path, added tests

Change of path:
  on some distributions, httpd has virtual /tmp/ therefore unix socket
  file cannot be found
Added tests:
  check if unix socket file exists, is readable and writable
Merge branch 'master' of github.com:CESNET/Netopeer-GUI