[sr-dev] [kamailio/kamailio] Support to get environment variable (Issue #3558)

Hi I'm aware that to get environment variable we can use `$env(varname)` however this require to load pv module first, eg: below:

``` loadmodule "pv" #!substdef "/DBRWUSER/$env(DBRWUSER)/g" #!substdef "/DBRWPW/$env(DBRWPW)/g" ```

in some case we need to get variable at the beginning (or convenient way or modern application), example:

``` alias=$getenv(DOMAIN) listen="$getenv(PRIVATE_IP)" advertise "$getenv(PUBLIC_IP)" ```

** $getenv just name of new function is similar to $env but not require pv