Add ExtraRootCAs *x509.CertPool to tsd.System and plumb it through
the control client, noise transport, DERP, and wgengine layers so
that platforms like Android can inject user-installed CA certificates
into Go's TLS verification.
tlsdial.Config now honors base.RootCAs as additional trusted roots,
tried after system roots and before the baked-in LetsEncrypt fallback.
SetConfigExpectedCert gets the same treatment for domain-fronted DERP.
The Android client will set sys.ExtraRootCAs with a pool built from
x509.SystemCertPool + user-installed certs obtained via the Android
KeyStore API, replacing the current SSL_CERT_DIR environment variable
approach.
Updates #8085
Change-Id: Iecce0fd140cd5aa0331b124e55a7045e24d8e0c2
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
a182b864ac