engine: resources: Improve the file res and add strict state

This might be slightly controversial, in that you must specify the state
if a file would need to be created to perform the action. We no longer
implicitly assume that just specifying content is enough. As it turns
out, I believe this is safer and more correct. The code to implement
this turns out to be much more logical and simplified, and does this
removes an ambiguous corner case from the reversed resource code.

Some discussion in: https://github.com/purpleidea/mgmt/issues/540

This patch also does a bit of related cleanup.

docs/faq.md

@@ -226,6 +226,34 @@ it and replace it with your git cloned directory. In my case, I like to work on
 things in `~/code/mgmt/`, so that path is a symlink that points to the long
 project directory.
 
+### Why does my file resource error with `no such file or directory`?
+
+If you create a file resource and only specify the content like this:
+
+```
+file "/tmp/foo" {
+	content => "hello world\n",
+}
+```
+
+Then this will attempt to set the contents of that file to the desired string,
+but *only* if that file already exists. If you'd like to ensure that it also
+gets created in case it is not present, then you must also specify the state:
+
+```
+file "/tmp/foo" {
+	state => "exists",
+	content => "hello world\n",
+}
+```
+
+Similar logic applies for situations when you only specify the `mode` parameter.
+
+This all turns out to be more safe and "correct", in that it would error and
+prevent masking an error for a situation when you expected a file to already be
+at that location. It also turns out to simplify the internals significantly, and
+remove an ambiguous scenario with the reversable file resource.
+
 ### On startup `mgmt` hangs after: `etcd: server: starting...`.
 
 If you get an error message similar to: