[Users] pseudo-variables notation update

[Daniel-Constantin Mierla]

Hello,

in order to allow names with different length, the pseudo-variables 
notation is similar with the one in make. You can enclose the name in 
between '(' and ')', e.g. $ru is the same with $(ru). This comes with 
the support of registering new pseudo-variables dynamically from 
modules. As an example, tm module exports $T_branch_idx which refers to 
the branch index for which is executed the branch_route[]. In case when 
parenthesis are not used, the name of the pseudo variable is considered 
to end at the first character which is not digit, letter, "." or "_".

Also, the notation for avp names has changed to avoid confusions 
(see:http://openser.org/pipermail/devel/2006-February/001943.html ). Now 
the following notation must be used:

- $avp(s:avp_name) - refers to the avp having the string name 'avp_string'
- $avp(i:avp_id) - refers to the avp having the integer id 'avp_id'
- $avp(avp_alias) - refers to the avp defined by the alias 'avp_alias'

Cheers,
Daniel