bookmarks: disallow bookmarks named 'tip', '.', or 'null' This makes bookmarks reject the same reserved names as tags and branches.

bookmarks: abort directly on invalid name This consolidates the abort message for a bookmark name containing invalid characters into one place, and renames the valid() method to checkvalid() to reflect the fact that it's no longer a predicate.

bookmarks: remove redundant check for newline New bookmarks are already checked for illegal characters (':', '\0', '\n', and '\r') in bookmarks.valid().

test-tag: test that all reserved names are rejected