MINOR: vars: store flags into variables and add VF_PERMANENT

MINOR: vars: store flags into variables and add VF_PERMANENT

In order to continue to honor the ifexist Lua option and prevent rogue
SPOA agents from creating too many variables, we'll need to keep the
ability to mark certain proc.* variables as permanent when they're
known from the config file.

Let's add a flag there for this. It's added to the variable when the
variable is created with this flag set by the caller.

Another approach could have been to use a distinct list or distinct
scope but that sounds complicated and bug-prone.