minijail: Expand is_canonical_path.

Some Minijail invocations use trailing slashes in -b paths. Since these
trailing slashes are not a risk symlink-wise, allow them for now while
we clean them up.

BUG=b:241137716
TEST=New unit tests.
TEST=login.Chrome, arc.Boot, security.Minijail*.

Change-Id: I8e49e5f551ed9d4ef61b38e62577183d36779030
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/minijail/+/3820909
Commit-Queue: Jorge Lucangeli Obes <[email protected]>
Reviewed-by: Ben Scarlato <[email protected]>
Tested-by: Jorge Lucangeli Obes <[email protected]>
Reviewed-by: Allen Webb <[email protected]>
diff --git a/system.h b/system.h
index fb351b4..11b7030 100644
--- a/system.h
+++ b/system.h
@@ -69,6 +69,7 @@
  * -Absolute.
  * -No symlinks.
  * -No /./, /../, or extra '/'.
+ * -Single trailing '/' is OK.
  */
 bool is_canonical_path(const char *path);