Reintroduce user impersonation from the command line, this time without messing with the cli syntax.

On startup, the realuser variable is set to :
  - first from the configuration file parameter 'realuser' if it is set
  - then from the environment variable 'NIPAP_REAL_USER' if it is set

Before running any modification command, the nipap username is set to the value of the realuser variable if it is set.
This username will be written in the ip_net_log database table on field username.
Allow smart searching on the vlan tag using the @ symbol
Update the 'search help' popup documentation for the vlan smart searching added in commit 979ef42
Add a new search function in the address cli in order to mimic the www GUI smart search field.

It is somewhat similar to the list function but it can return only one result.
A script boolean option is also added to only print the result display_prefix

A sample use is : nipap address search tags GW vlan 666 script true or nipap address search 192.168.0.1/32 tags DMZ,PROD