Contining the trend of making everything be optional that could
possibly be optional, this makes the magicsock UDP underlay transport
be optional. (mostly replacing a bunch of GOOS != "js" checks in the process)
Omitting udptransport (ts_omit_udptransport) makes magicsock DERP-only: no
UDP sockets, no advertised endpoints, and netcheck measures DERP latency
over HTTPS. Omitting only nattraversal keeps disco ping/pong to peers'
advertised endpoints but drops STUN, call-me-maybe, the peer relay client,
the endpoint tracker, and UDP lifetime probing.
Both are plain linker dead-code elimination via buildfeatures
constants. (as oppposed to moving the code all over into feature
packages and indirecting through hooks) The minimal linux/amd64
tailscaled shrinks by 258 KB without nattraversal and 561 KB without
both.
Updates #12614
Change-Id: Ife10804b48a168d9481e4321c18ed8fc85fa71f1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Brad Fitzpatrick
fe91043315