Add a notion of "physical" sysroot, use for remote writing

(Note this PR was reverted in <https://github.com/ostreedev/ostree/pull/902>;
 this version should be better)

Using `${sysroot}` to mean the physical storage root: We don't want to write to
`${sysroot}/etc/ostree/remotes.d`, since nothing will read it, and really
`${sysroot}` should just have `/ostree` (ideally). Today the Anaconda rpmostree
code ends up writing there. Fix this by adding a notion of "physical" sysroot.
We determine whether the path is physical by checking for `/sysroot`, which
exists in deployment roots (and there shouldn't be a `${sysroot}/sysroot`).

In order to unit test this, I added a `--sysroot` argument to `remote add`.
However, doing this better would require reworking the command line parsing for
the `remote` argument to support specifying `--repo` or `--sysroot`, and I
didn't quite want to do that yet in this patch.

This second iteration of this patch fixes the bug we hit the first time;
embarassingly enough I broke `ostree remote list` finding system remotes.
The fix is to have `ostree_repo_open()` figure out whether it's the same
as `/ostree/repo` for now.

Down the line...we might consider having the `ostree remote` command line itself
instatiate an `OstreeSysroot` by default, but this maximizes compatibility; we
just have to pay a small cost that `ostree` usage outside of that case like
`ostree static-delta` in a releng Jenkins job or whatever will do this `stat()`
too.

Closes: https://github.com/ostreedev/ostree/issues/892

Closes: #1008
Approved by: mbarnes

src/libostree/ostree-repo-private.h

@@ -80,6 +80,13 @@ struct OstreeRepoCommitModifier {
   GHashTable *devino_cache;
 };
 
+typedef enum {
+  OSTREE_REPO_SYSROOT_KIND_UNKNOWN,
+  OSTREE_REPO_SYSROOT_KIND_NO,  /* Not a system repo */
+  OSTREE_REPO_SYSROOT_KIND_VIA_SYSROOT, /* Constructed via ostree_sysroot_get_repo() */
+  OSTREE_REPO_SYSROOT_KIND_IS_SYSROOT_OSTREE, /* We match /ostree/repo */
+} OstreeRepoSysrootKind;
+
 /**
  * OstreeRepo:
  *
@@ -101,6 +108,7 @@ struct OstreeRepo {
   int objects_dir_fd;
   int uncompressed_objects_dir_fd;
   GFile *sysroot_dir;
+  GWeakRef sysroot; /* Weak to avoid a circular ref; see also `is_system` */
   char *remotes_config_dir;
 
   GHashTable *txn_refs;  /* (element-type utf8 utf8) */
@@ -118,7 +126,7 @@ struct OstreeRepo {
 
   gboolean inited;
   gboolean writable;
-  gboolean is_system; /* Was this repo created via ostree_sysroot_get_repo() ? */
+  OstreeRepoSysrootKind sysroot_kind;
   GError *writable_error;
   gboolean in_transaction;
   gboolean disable_fsync;