lib/repo: Add min-free-space-percent option, default 3%

For ostree-as-host, we're the superuser, so we'll blow past
any reserved free space by default.  While deltas have size
metadata, if one happens to do a loose fetch, we can fill
up the disk.

Another case is flatpak: the system helper has similar concerns
here as ostree-as-host, and for `flatpak --user`, we also
want to be nice and avoid filling up the user's quota.

Closes: https://github.com/ostreedev/ostree/issues/962

Closes: #987
Approved by: jlebon

src/libostree/ostree-repo.c

@@ -44,6 +44,7 @@
 #include <locale.h>
 #include <glib/gstdio.h>
 #include <sys/file.h>
+#include <sys/statvfs.h>
 
 /**
  * SECTION:ostree-repo
@@ -1964,6 +1965,20 @@ reload_core_config (OstreeRepo          *self,
       self->zlib_compression_level = OSTREE_ARCHIVE_DEFAULT_COMPRESSION_LEVEL;
   }
 
+  { g_autofree char *min_free_space_percent_str = NULL;
+    /* If changing this, be sure to change the man page too */
+    const char *default_min_free_space = "3";
+
+    if (!ot_keyfile_get_value_with_default (self->config, "core", "min-free-space-percent",
+                                            default_min_free_space,
+                                            &min_free_space_percent_str, error))
+      return FALSE;
+
+    self->min_free_space_percent = g_ascii_strtoull (min_free_space_percent_str, NULL, 10);
+    if (self->min_free_space_percent > 99)
+      return glnx_throw (error, "Invalid min-free-space-percent '%s'", min_free_space_percent_str);
+  }
+
   {
     g_clear_pointer (&self->collection_id, g_free);
     if (!ot_keyfile_get_value_with_default (self->config, "core", "collection-id",