Make P2P API public (no longer experimental)

Currently the API that allows P2P operations (e.g. pulling an ostree ref
from a LAN or USB source) is hidden behind the configure flag
--enable-experimental-api. This commit makes the API public and makes
that flag essentially a no-op (leaving it in place in case we want to
use it again in the future). The P2P API has been tested over the last
several months and proven to work.

This means that since we're no longer using the "experimental" feature
flag, P2P builds of Flatpak will fail when using versions of OSTree from
this commit onwards, until Flatpak is patched in the near future. If you
want to build Flatpak < 0.11.8 with P2P enabled and link against OSTree
2018.6, you'll have to patch Flatpak.  However, since Flatpak won't yet
have a hard dependency on OSTree 2018.6, it needs a new way to determine
if the P2P API in OSTree is available, so this commit adds a "p2p"
feature flag. This way the feature set is more semantically correct than
if we had continued to use the "experimental" feature flag.

In addition to making the P2P API public, this commit makes the P2P unit
tests run by default, removes the f27-experimental CI instance that's no
longer needed, changes a few man pages to reflect the changes, and
updates the bash completion script to accept the new commands and
options.

Closes: #1596
Approved by: cgwalters

src/libostree/ostree-repo-refs.c

@@ -470,7 +470,6 @@ ostree_repo_resolve_rev_ext (OstreeRepo                    *self,
   return _ostree_repo_resolve_rev_internal (self, refspec, allow_noent, FALSE, out_rev, error);
 }
 
-#ifdef OSTREE_ENABLE_EXPERIMENTAL_API
 /**
  * ostree_repo_resolve_collection_ref:
  * @self: an #OstreeRepo
@@ -494,7 +493,7 @@ ostree_repo_resolve_rev_ext (OstreeRepo                    *self,
  * There are currently no @flags which affect the behaviour of this function.
  *
  * Returns: %TRUE on success, %FALSE on failure
- * Since: 2017.12
+ * Since: 2018.6
  */
 gboolean
 ostree_repo_resolve_collection_ref (OstreeRepo                    *self,
@@ -531,7 +530,6 @@ ostree_repo_resolve_collection_ref (OstreeRepo                    *self,
     *out_rev = g_strdup (ret_contents);
   return TRUE;
 }
-#endif  /* OSTREE_ENABLE_EXPERIMENTAL_API */
 
 static gboolean
 enumerate_refs_recurse (OstreeRepo    *repo,
@@ -876,7 +874,6 @@ ostree_repo_remote_list_refs (OstreeRepo       *self,
   return TRUE;
 }
 
-#ifdef OSTREE_ENABLE_EXPERIMENTAL_API
 static gboolean
 remote_list_collection_refs_process_refs (OstreeRepo   *self,
                                           const gchar  *remote_name,
@@ -934,7 +931,7 @@ remote_list_collection_refs_process_refs (OstreeRepo   *self,
  * Any refs for other collections stored in the repository will also be returned.
  * No filtering is performed.
  *
- * Since: 2017.10
+ * Since: 2018.6
  */
 gboolean
 ostree_repo_remote_list_collection_refs (OstreeRepo    *self,
@@ -995,7 +992,6 @@ ostree_repo_remote_list_collection_refs (OstreeRepo    *self,
   ot_transfer_out_value (out_all_refs, &ret_all_refs);
   return TRUE;
 }
-#endif  /* OSTREE_ENABLE_EXPERIMENTAL_API */
 
 static char *
 relative_symlink_to (const char *relpath,
@@ -1206,7 +1202,7 @@ _ostree_repo_update_collection_refs (OstreeRepo        *self,
  * %OSTREE_REPO_LIST_REFS_EXT_EXCLUDE_REMOTES in @flags.
  *
  * Returns: %TRUE on success, %FALSE otherwise
- * Since: 2017.8
+ * Since: 2018.6
  */
 gboolean
 ostree_repo_list_collection_refs (OstreeRepo                 *self,