Improve handling of leading and trailing spaces in configuration files

Current situation

Configuration parsing is quite strict wrt spaces (in configuration files).

For example the following config entry:

/etc/squashtf/admin_trusted_key.pub, admin, groupe_admin, "admin"

gives value admin for column 2, groupe_admin for column 3, and "admin" for column 4.

Desired outcome

Being able to add leading or trailing spaces in configuration files.