Instead of lengthy blurbs, switch to single-line, machine-readable
standardized (https://spdx.dev) license identifiers. The Linux kernel
switched long ago, so there is strong precedent.
Three cases are handled: AGPL-only, Apache-only, and dual licensed.
For the latter case, I chose (AGPL-3.0-or-later and Apache-2.0),
reasoning that our changes are extensive enough to apply our license.
The changes we applied mechanically with a script, except to
licenses/README.md.
Closes#9937
The set of allowed options is quite small, so we benefit from a static
representation (member variables) over a dynamic map.
We also logically move the "OPTIONS" option to the domain of the
authenticator (from user management), since this is where it is applied.
This refactor also aims to reduce compilation time by moving
`authentication_options` into its own header file.
While changes to `user_options` were necessary to accommodate the new
structure, that class will be deprecated shortly in the switch to roles.
Therefore, the changes are strictly temporary.
