rules: minor code reordering

Improve code order by putting code testing for archive at first.
rules: add individual sigs file support

This patch adds support for the handling of rules file that can
be downloaded or uploaded.
rules: relax parsing of signatures
suricata: fix edit function
rules: add capabilities to update sources list

It is now possible to update the list of sources used by a ruleset.
This allows to add new sources without having to setup again the
existing sources.
doc: sync with new features

The support for individual files and sources edition were not
explained in the doc.