Add ability to read rules files from directories (#348)

* Add ability to read rules files from directories

When the argument to -r <path> or an entry in falco.yaml's rules_file
list is a directory, read all files in the directory and add them to the
rules file list. The files in the directory are sorted alphabetically
before being added to the list.

The installed falco adds directories /etc/falco/rules.available and
/etc/falco/rules.d and moves /etc/falco/application_rules.yaml to
/etc/falco/rules.available. /etc/falco/rules.d is empty, but the idea is
that admins can symlink to /etc/falco/rules.available for applications
they want to enable.

This will make it easier to add application-specific rulesets that
admins can opt-in to.

* Unit test for reading rules from directory

Copy the rules/trace file from the test multiple_rules to a new test
rules_directory. The rules files are in rules/rules_dir/{000,001}*.yaml,
and the test uses a rules_file argument of rules_dir. Ensure that the
same events are detected.

test/rules/rules_dir/000-single_rule.yaml

@@ -0,0 +1,14 @@
+- list: cat_binaries
+  items: [cat]
+
+- list: cat_capable_binaries
+  items: [cat_binaries]
+
+- macro: is_cat
+  condition: proc.name in (cat_capable_binaries)
+
+- rule: open_from_cat
+  desc: A process named cat does an open
+  condition: evt.type=open and is_cat
+  output: "An open was seen (command=%proc.cmdline)"
+  priority: WARNING

test/rules/rules_dir/001-double_rule.yaml

@@ -0,0 +1,13 @@
+# This ruleset depends on the is_cat macro defined in single_rule.yaml
+
+- rule: exec_from_cat
+  desc: A process named cat does execve
+  condition: evt.type=execve and is_cat
+  output: "An exec was seen (command=%proc.cmdline)"
+  priority: ERROR
+
+- rule: access_from_cat
+  desc: A process named cat does an access
+  condition: evt.type=access and is_cat
+  output: "An access was seen (command=%proc.cmdline)"
+  priority: INFO