SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
According to RFC6066, empty extension_data for an SNI extension is
absolutely one of the possibilities - so let's support this instead of
spamming the debug log.
The user may now request specific cipher suites, the use of SNI, and
whether we should validate certificates (not that we're doing a good job
of that).
