This disallows labels that start with digits (as per RFC 1035 section
2.3.1). The invalid character error text is also improved so that UTF-8
sequences are properly copied to the error rather than an encoding of
the first byte.
Updates #13858
Change-Id: Ib3fea46559918e7bba6b11c1ca33ede61755bd52
Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>