pfSense

Introduce read_global_var:

- Add a new script, read_global_var, that returns the value of any item
contained in global array $g
- Replace all grep and sed magic from shell scripts

Introduce read_xml_tag.sh

Create /usr/local/sbin/read_xml_tag.sh, that will read a xml tag from
config file using xmllint. Replace all direct read using cat, grep, sed,
... by this script

Consider globals_override.inc and replace hard coded pfSense string by product_name

Move main pfSense content to src/