MEDIUM: vars: Add the set-vars-from-map action

MEDIUM: vars: Add the set-vars-from-map action

Add a new action, set-vars-from-map(<scope>,<map-file>[,<type>]), that
loads a two-column (key/value) map file and sets one variable per entry,
named "<scope>.<key>", to the entry's value. The scope is one of sess,
txn, req, res or check and the variable name is relative to it.
The new "tune.vars.from-map.max" global option defines the maximum
number of entries taken into account in each map. It default to 100.

Values are read as strings and converted according to the optional
<type>, which may be "str" (default), "int" or "bool". Integer
conversion is strict, so an entry with trailing garbage is rejected
rather than silently truncated, and booleans accept 0/1 as well as
true/false in any case. An entry that cannot be parsed is skipped
without aborting the remaining entries.

The file is loaded as a pattern reference flagged PAT_REF_SMP, or reused
if it was already loaded, in which case it must also have been loaded as
a two-column file. The action is registered for all rule sets that
already support set-var.

This was GH issue #1619.