net/tlsdial: bake in LetsEncrypt's ISRG Root X1 root

We still try the host's x509 roots first, but if that fails (like if
the host is old), we fall back to using LetsEncrypt's root and
retrying with that.

tlsdial was used in the three main places: logs, control, DERP. But it
was missing in dnsfallback. So added it there too, so we can run fine
now on a machine with no DNS config and no root CAs configured.

Also, move SSLKEYLOGFILE support out of DERP. tlsdial is the logical place
for that support.

Fixes #1609

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

go.sum

@@ -14,6 +14,8 @@ cloud.google.com/go/firestore v1.1.0/go.mod h1:ulACoGHTpvq5r8rxGJ4ddJZBZqakUQqCl
 cloud.google.com/go/pubsub v1.0.1/go.mod h1:R0Gpsv3s54REJCy4fxDixWD93lHJMoZTyQ2kNxGRt3I=
 cloud.google.com/go/storage v1.0.0/go.mod h1:IhtSnM/ZTZV8YYJWCY8RULGVqBDmpoyjwiyrjsg+URw=
 dmitri.shuralyov.com/gpu/mtl v0.0.0-20190408044501-666a987793e9/go.mod h1:H6x//7gZCb22OMCxBHrMx7a5I7Hp++hsVxbQ4BYO7hU=
+filippo.io/mkcert v1.4.3 h1:axpnmtrZMM8u5Hf4N3UXxboGemMOV+Tn+e+pkHM6E3o=
+filippo.io/mkcert v1.4.3/go.mod h1:64ke566uBwAQcdK3vRDABgsgVHqrfORPTw6YytZCTxk=
 github.com/BurntSushi/toml v0.3.1 h1:WXkYYl6Yr3qBf1K79EBnL4mak0OimBfB0XUf9Vl28OQ=
 github.com/BurntSushi/toml v0.3.1/go.mod h1:xHWCNGjB5oqiDr8zfno3MHue2Ht5sIBksp03qcyfWMU=
 github.com/BurntSushi/xgb v0.0.0-20160522181843-27f122750802/go.mod h1:IVnqGOEym/WlBOVXweHU+Q+/VP0lqqI8lqeDx9IjBqo=
@@ -876,6 +878,7 @@ golang.org/x/tools v0.0.0-20201011145850-ed2f50202694/go.mod h1:z6u4i615ZeAfBE4X
 golang.org/x/tools v0.0.0-20201013201025-64a9e34f3752/go.mod h1:z6u4i615ZeAfBE4XtMziQW1fSVJXACjjbWkB/mvPzlU=
 golang.org/x/tools v0.0.0-20201118003311-bd56c0adb394/go.mod h1:emZCQorbCU4vsT4fOWvOPXz4eW1wZW4PmDk9uLelYpA=
 golang.org/x/tools v0.0.0-20201121010211-780cb80bd7fb/go.mod h1:emZCQorbCU4vsT4fOWvOPXz4eW1wZW4PmDk9uLelYpA=
+golang.org/x/tools v0.0.0-20201124202034-299f270db459/go.mod h1:emZCQorbCU4vsT4fOWvOPXz4eW1wZW4PmDk9uLelYpA=
 golang.org/x/tools v0.0.0-20201208233053-a543418bbed2/go.mod h1:emZCQorbCU4vsT4fOWvOPXz4eW1wZW4PmDk9uLelYpA=
 golang.org/x/tools v0.0.0-20201211185031-d93e913c1a58/go.mod h1:emZCQorbCU4vsT4fOWvOPXz4eW1wZW4PmDk9uLelYpA=
 golang.org/x/tools v0.1.0/go.mod h1:xkSsbof2nBLbhDlRMhhhyNLN/zl3eTqcnHD5viDpcZ0=
@@ -957,6 +960,8 @@ honnef.co/go/tools v0.0.1-2019.2.3/go.mod h1:a3bituU0lyd329TUQxRnasdCoJDkEUEAqEt
 honnef.co/go/tools v0.0.1-2020.1.6/go.mod h1:pyyisuGw24ruLjrr1ddx39WE0y9OooInRzEYLhQB2YY=
 honnef.co/go/tools v0.2.1 h1:/EPr//+UMMXwMTkXvCCoaJDq8cpjMO80Ou+L4PDo2mY=
 honnef.co/go/tools v0.2.1/go.mod h1:lPVVZ2BS5TfnjLyizF7o7hv7j9/L+8cZY2hLyjP9cGY=
+howett.net/plist v0.0.0-20181124034731-591f970eefbb h1:jhnBjNi9UFpfpl8YZhA9CrOqpnJdvzuiHsl/dnxl11M=
+howett.net/plist v0.0.0-20181124034731-591f970eefbb/go.mod h1:vMygbs4qMhSZSc4lCUl2OEE+rDiIIJAIdR4m7MiMcm0=
 inet.af/netaddr v0.0.0-20210515010201-ad03edc7c841/go.mod h1:z0nx+Dh+7N7CC8V5ayHtHGpZpxLQZZxkIaaz6HN65Ls=
 inet.af/netaddr v0.0.0-20210721214506-ce7a8ad02cc1 h1:mxmfTV6kjXTlFqqFETnG9FQZzNFc6AKunZVAgQ3b7WA=
 inet.af/netaddr v0.0.0-20210721214506-ce7a8ad02cc1/go.mod h1:z0nx+Dh+7N7CC8V5ayHtHGpZpxLQZZxkIaaz6HN65Ls=
@@ -976,3 +981,5 @@ mvdan.cc/lint v0.0.0-20170908181259-adc824a0674b/go.mod h1:2odslEg/xrtNQqCYg2/jC
 mvdan.cc/unparam v0.0.0-20200501210554-b37ab49443f7 h1:kAREL6MPwpsk1/PQPFD3Eg7WAQR5mPTWZJaBiG5LDbY=
 mvdan.cc/unparam v0.0.0-20200501210554-b37ab49443f7/go.mod h1:HGC5lll35J70Y5v7vCGb9oLhHoScFwkHDJm/05RdSTc=
 rsc.io/binaryregexp v0.2.0/go.mod h1:qTv7/COck+e2FymRvadv62gMdZztPaShugOCi3I+8D8=
+software.sslmate.com/src/go-pkcs12 v0.0.0-20180114231543-2291e8f0f237 h1:iAEkCBPbRaflBgZ7o9gjVUuWuvWeV4sytFWg9o+Pj2k=
+software.sslmate.com/src/go-pkcs12 v0.0.0-20180114231543-2291e8f0f237/go.mod h1:/xvNRWUqm0+/ZMiF4EX00vrSCMsE4/NHb+Pt3freEeQ=