This updates all source files to use a new standard header for copyright
and license declaration. Notably, copyright no longer includes a date,
and we now use the standard SPDX-License-Identifier header.
This commit was done almost entirely mechanically with perl, and then
some minimal manual fixes.
Updates #6865
Signed-off-by: Will Norris <will@tailscale.com>
It was accidentally deleted in the earlier 0022c3d2e (#2143) refactor.
Lock it in with a test.
Fixestailscale/corp#2503
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
